I think your definition of underpowered is stupidly incorrect, but you still do make a point in the grander picture here.
I mean I don't play certain champs because I don't find it fun to do so, so if someone on the other team gives me a champ that I never play because I don't enjoy, I'm still not going to enjoy it! Nemesis draft is just generally an awful idea and I whole heartedly think Riot is just trying to get everyone to play the less played champs to try and bring them back into the game without Riot having to change them directly.
Anyway yeah you're wrong about those champs, Zac is a very strong tank jungler with ridiculous sustain and clear, Shen has always been AT LEAST average in top lane, if Poppy can get a good match up in top lane she can wreck carries late game, Yorick still has very few lane counters, Garen is actually very strong early game, Alistair has been a strong pick for a long time now, Mundo is extremely effective in certain comps, and Rammus is actually one of the strongest tank junglers atm.
But yeah Taric is awful :P