A+ -> A
Don't take this the wrong way: Machoke is still amazing for Bulky Offense and Balance in the ZU meta. However, with the addition of numerous mons that can dish out a lot of super effective damage to it or take advantage of it (Farfetch'd, Gothitelle, Mr. Mime, Mr. Mime-Galar, Shiinotic), it's lost its position as meta-defining. Machoke is still great at giving defensive Eviolite users grief, threatening staples such as Persian and Linoone-Galar, and putting the pressure on Stall teams, but it does not enjoy this recent surge in powerful special attackers that coincidentally can dish out lots of damage against it.
A+ -> A
Wartortle coincidentally suffers Machoke's same fate of losing its luster in the face of drops, but less due to threatening (though that is part of it) and more due to role competition. Mr. Mime-Galar being in the tier now means that Wartortle's only competition as a Spinner isn't Delibird anymore (foreshadowing noises). On top of that, Mr. Mime-Galar is also a setup sweeper that isn't completely walled by Water types and has a significantly better Special Attack stat. Wartortle is still good, but it isn't at the very top anymore, and dropping it at least one subrank should reflect that.
A- -> B+
Lunatone may have lost a lot of its checks and counters over the past month (Thwackey, Thievul, Silvally-Grass), but it has unfortunately gained both new checks as well as competitors for its role. Gourgeist-Large is quite dominant right now, Shiinotic has the bulk to take hits and recover it off, and even Farfetch'd threatens it with First Impression (which ties into Lunatone's weakness to the surge in priority like Machoke's Bullet Punch, Beartic's Aqua Jet and various mons' Shadow Sneak). Its Scarf set is less useful thanks to the presence of Mr. Mime as a stronger and faster Psychic type Scarfer that offers greater utility options. Lunatone's rocks set suffers from the introduction of Krokorok, which dominates it with Dark STAB and Crunch. While Meteor Beam and Power Herb is a fine choice on HO, there are other special setup sweepers that can offer better STAB options, and Lunatone is just too frail, slow, and has a poor typing for overcoming the various priority running around at the moment to hit the Mimes. Overall, I don't think Lunatone really is worthy of being in the A ranks anymore, as its sets have lost their splashability and its type is more of a liability in the current meta.
A- -> B+
Another victim of the grassy surge that continues on even without Thwackey's presence, Whiscash is just not enjoying the current state of affairs in ZU. Its offensive stats are still pretty paltry, making Specs have to predict extremely well or giving Dragon Dance a harder time finding a sweep. It definitely does not help that even at +1 it has to either run Jolly or risk a speed tie with a Mr. Mime-G that can OHKO it with a Freeze-Dry. Pyukumuku, Shiinotic, Dusclops, and Shedinja all stop its sweeps cold, are pretty popular at the moment, and have a nasty habit of staying alive to the end of the game. Whiscash just does not cut it anymore, and A- was still pretty generous for it in a meta that had Pincurchin.
A- -> B+
Silvally-Ice is a peculiar case for me. On one hand, it is still a Silvally that can SD up and break things apart with a 120 BP STAB Multi-Attack with no drawbacks. On the other hand, Silvally-Ice's viability rank comes from a time where there were not one, not two, but three other Silvally in the tier. The unpredictability factor it once had has been diminished a bit. Along with that, the 95 base speed benchmark it once had is no longer the shining jewel of ZU now that fast offensive threats have made their mark on the meta, and priority is everywhere. Icevally does not appreciate hazards in the slightest, but running the amazing new Mime-G as removal for it stacks a lot of weaknesses. Ice is currently a pretty good offensive typing in a meta with lots of Grass in it, but is such a liability defensively, and Silvally-Ice often takes more chip from hazards and priority than it would like. If setup wasn't difficult enough, Dusclops and Pyukumuku have returned to make stall nasty again, which means that Icevally's SD set is less good at breaking than it once was. Icevally just isn't having a good moment right now, and dropping a rank is fitting for that.
B+ -> B
While Ivysaur can offer some good utility against Stall between its typing and access to Knock Off, it struggles a lot with some of the new faces in the meta. The Mimes, Farfetch'd, and Gothitelle all make its life difficult and the trend towards more powerful special attackers, particularly Ice and Psychic, means that its typing doesn't look as good anymore. Shiinotic competes with Ivysaur's role as a defensive Grass type, and even carries the extremely reliable Spore to shut down foes more consistently than Sleep Powder. Gloom has not been doing as hot either, and I believe that if it drops, Ivysaur should also go down a notch.
B+ -> B
I was never really that hot on Maractus to begin with, but the new meta trends have definitely made its viability placement much more suspect. Farfetch'd's First Impression straight up outprioritizes Sucker Punch, and Maractus also has to deal with the sudden prevalence of faster attackers that all take advantage of its bad bulk and can outplay some of its Sucker Punches. While it was pretty good as a mix of anti-offense and stallbreaking, it really did not like that the Mimes and Shiinotic dropped into the meta. Maractus is still a great stallbreaker that can dismantle Pyuklops to some extent, it struggles to keep up in the current meta with its mediocre offensive typing and the speed and priority creep ZU is facing.
B -> B-
Rain was already a very shaky archetype in ZU, but reliance on Vulpix-A or Snover as pure setters as Eiscue's Hail set declines is just not great for Beartic. It's still literally the most physically powerful mon in ZU before taking into account that it can run CB or SD, but it doesn't have as easy of a time terrorizing a faster meta that isn't just Eviolite walls anymore. Pyukumuku and Dusclops ruin its stall matchup if it doesn't run Taunt, while the priority that has become prominent in the meta to hit Mime-G has made its life worse. Beartic just isn't the giant it once was.
B -> B-
I think Duosion is in a weird place right now. Its offensive Trick Room set competes somewhat with the popularity of Oranguru, it hates the additions of Krokorok and Poltergeist, and there's new Psychic types that can function as wincons (Gothitelle, Mr. Mime, Mr. Mime-Galar) that give it competition. Even more frustrating for it is that Dusclops is another bulky defensive booster that can end games. While its sweeper set it still fine and is excellent at being a wincon on semi-stall and against Pyukumuku, Duosion just doesn't really fit the current meta. Kind of sad, as I love the little blob :(
B -> B-
While you might think that Gastly's Scarf set would thrive in a meta of high speed Psychic types to revenge kill, things are never that simple. Shadow Sneaking ghosts like Gourg-L and Shedinja give Gastly grief, and it can't come in to spinblock Mr. Mime-G for fear of getting hit with a Psychic. Gastly just does not distinguish itself in a meta that is swimming in priority, Knock Off users, and better spinblocking Ghosts.
B -> B-
Want a fast rocker that can run Taunt? Krokorok is the pick for you! Onix does have Body Press and Head Smash still to guarantee rocks as a lead, but it has now lost its big title of "fastest rocker who comes prepackaged with Taunt".
B -> B-
Scraggy being not just viable but good has been very fun, and it was great for having the bulk to pull off multiple distinct setup sets that could capitalize on the lower speed of the meta and a shortage of viable strong Fighting, Flying, and Fairy types to break through it. Unfortunately, those days are now gone, as Shiinotic, Mr. Mime, and Farfetch'd have come to ruin its day. The return of Pyukumuku hasn't made its setup life any easier as well. Togetic may be gone, but more threats to Scraggy's success have popped up in its place.
B- -> C
Being a Magic Bouncer is much more fun when everything wasn't out to get you. Natu's bulk was already flimsy and compromised both its defensive and Wishporting sets, but now it has to deal with the fact that Krokorok is a thing, many popular old hazard setters (Glalie, Maractus, Lunatone, Solrock) still beat it, and there's suddenly a lot of things running Knock Off or Poltergeist. Sure it can shut down Pyukumuku, but Natu's stallbreaking ends when Dusclops or any other Ghost comes in.
C -> UR
How the mighty fall, and how the not so mighty fall even harder. Cherrim's only claim to fame since the Drought + Heat Rock transitive ban is its access to Healing Wish, which was the only reason one would run a Scarf Cherrim. With Mr. Mime (and its Galarian counterpart, but don't run Healing Wish on that) entering the tier and bearing its own Scarf set with much greater coverage, better power, and a higher speed tier, there's very little reason to run Cherrim anymore.
C -> UR
Delibird was already fringe due to its bad bulk, low power without the inconsistent Hustle, and mediocre speed even by ZU standards. Heavy-Duty Boots were barely enough to save it, but the drop of Mr. Mime-Galar has made its role as a Rapid Spinner that is not Wartortle evaporate. If you want an Ice Type Spiker that can run a lead set, Glalie is a much more consistent option. If you want a fragile but potent Ice type Spinner, go with Mr. Mime-Galar. Delibird's negative qualities just make it too weak of a compromise between two different roles.
C -> UR
While Munchlax was a fine special wall that could phaze and occasionally set up with Curse, it suffers due to the readdition or strengthening of physical ghosts that wall and threaten it, such as Dusclops, Gourgeist-Large, and Shedinja. Munchlax grants free switches to these three in a way that compromises its ability to take special hits and then Rest off the damage. Being a momentum drain in a meta that has gotten faster and more specially powerful has hurt it as well, and I think that these factors make it inappropriate compared to some of the other niche presences in C.
C -> UR
Spritzee was, like, fine I guess in a meta where Wish support was limited to itself and Natu, but Lickitung's release has put it out of business in my opinion. Its overall stats are just too low to really justify it as a passer anymore. It has a fine defensive typing, but Lickitung offers the far more valuable Ghost immunity and Knock Off support. ZU society has progressed beyond the need for Spritzee.