Are you guys hearing yourself now? You are completely and utterly missing the point I'm trying to make. Lets take a look at these counters, because yes,
there are counters to Whimsicott, but very, very few of these counters are viable. Lets take a look at these soundproof mons.
Abomasnow
Bastiodon
Electrode
Mr. Mime
Bouffalant
Exploud
You know what? Only one of those pokemon is RU. The rest are PU, or in the case of Abomosnow which is PU if you don't use Snow Warning on it. Now lets take a look at Insomnia mons, actually lol, lets just list the mons who aren't PU
Honchkrow
...
Honchkrow dies to Moonblast so yeah, lets not.
Now lets do Sap sipper
Azumarill
Goodra
Bouffalant
Miltank
...
Only one of these pokemon are viable, Goodra, which can be widdled by Moonblast eventually but is otherwise a counter, and yes, sorry, I forgot about the other mons mentioned. You now have one counter to Whimsicott and a few other threats, however... When you actually look at Goodra it receives heavy competition from a plethora of walls in the tier, it's only real niche over things like Chansey is the ability to counter Whimsicott and Serperior.
Magic coat... Just... no? No pokemon, in any tier, runs Magic coat viable. Atleast nothing relevant. What are you even running it on?
Bug buzz? What? How is this any different from any other sound move, you realise both of these bug buzzers are just put to sleep? Yeah, Whimsicott isn't going to stay in but god damn.
252 SpA Volcarona Bug Buzz vs. 0 HP / 4 SpD Whimsicott: 192-226 (73.5 - 86.5%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Life Orb Yanmega Bug Buzz vs. 0 HP / 4 SpD Whimsicott: 246-290 (94.2 - 111.1%) -- 68.8% chance to OHKO
Both of these pokemon are put to sleep, and then Whimsicott switches into the appropriate counters. Not to mention that these are pretty... unviable. Yanmega isn't even ranked in OU while Volcarona is easy to handle with Tornadus-Therian.
Magic guard: Again, put to sleep and u-turn out, which is the same thing it does with every other thing that isn't beaten by leech seed.
Taunt Whimsicott has access to the fastest taunt in the game. You're not taunting it if it doesn't want to.
Thundurus is either put to sleep, or it's subbed on a predicted T-wave and then Leech seeded.
Talonflame Put to sleep on the switch
Mega Sableye if Mega Sableye is running some sp.def investment with Calm mind it's a counter
Rest talk Taunt. Sub. Leech seed.
Eevee General No vital spirit user even surpasses NU so I didn't really think they were relevant, all of them are beaten if Whimsicott leech seeds them anyways.
Verdict
I can't say it's broken, it doesn't 6-0 teams on its own, nor is it an insane power house that breaks everything. But it is centralising as shit, it's literally resorting you to use
magic coat or sleep talk on Tornadus-Therian. It is centralising and it provides pretty darn immense supportive capabilities, while it can 6-0 unprepared teams and stops sweeps cold.