Is Giovanni the most underwhelming gym leader of all time?
Ground is one of the least optimal types that a gym leader can specialize in for gen 1, and it becomes even less optimal for a late-game boss specifically. It's disadvantaged against all 3 starters. (Venusaur and Blastoise shred it; Charizard has ground immunity.) FRLG fleshes out the movesets slightly compared to gen 1 but counteracts that by bafflingly de-evolving his Rhydon.
Yes, because of the buildup. If Giovanni was just an average gym leader, nothing special about him, then he would be slightly underwhelming, but not the most underwhelming I believe (idk what would be the most underwhelming otherwise). However, this is not just an average gym leader, this is the leader of Team Rocket, the evil organisation that you have been contesting with for a while. You have faced him two times before, once at the Game Corner and once at Silph Co. (i don't think there was a third time). Team Rocket you have had multiple encounters with and revealing that the Gym Leader is Team Rocket's leader should make for a hype match.
And then you click surf and win.
Yeah, literally one of the easiest battles in the game. Even if they had optimised movesets, the team is just so bad. In Pokemon Yellow they gave him a Persian, which had autocrit slash which was really cool, but the team is still so bad, and they revert it for every other battle. Ground types in Gen 1 are slow, with the exception of dugtrio who hits limply. Fun fact, in RB none of his mons have a rock type move, in Yellow only Rhydon does and in FRLG the two rhyhorns only have rock blast, so flying types eat them up which you will have on your team because fly is really good. It's honestly pathetic.
One thing I think they should have done is included Kangaskhan (and Persian maybe) in the final team. He uses it in both prior battles, and with something like Shadow Ball or E-quake, it could have been a threat. E-quake would especially make sense since that's the TM he gives out.
Giovanni is the one gym leader that could have broken the rule of "only one type" that gym leaders stick to religiously (which is a bad thing, Volkner in DP is proof that you can make these teams work, though DP also shows how not to do that in Flint) due to him being a mob boss, but they refused once again because they wanted it to be all ground.