1) Thundercats was shit. Those of us who were in the target demo at the target time acknowledge that Thundercats was pure shit. You could argue about GI Joe and Transformers and Robotech but they were all acknowledged as better than Gobots, which was acknowledged as better than Masters of the Universe, which was acknowledged as better than Thundercats, which was arguably better than Tranzor Z, but not by much.
2) Shit as it was,
Go ahead and google image search "cheetara." You will note that fan images outnumber actual images by 10:1. If your safesearch is off, Rule 34 will clobber you. Despite the fact that Thundercats was acknowledged as pure shit, it still got your preadolescent tinglies on. It's fair to say that the ideation of Thundercats has moved far beyond the actuality of Thundercats and that ideation does not look like some goddamn Steamboat Willie lumpfest from the school of "model everything as turds."
3) Nerds will be nerds but there's the rippled-flesh nerds and there's the endomorphic "don't remind me of my body" nerds. The former favor AoT and Dragonball. The latter favor Adventure time and Steven Universe.
4) Thundercats was not the first school-of-GNC animation series (He Man beat them by two years). However, the School of Pillsbury would never dare to lay claim to He Man. Lion-O turning into Newman from Seinfeld? That's a betrayal of the principles of fandom.
For the record: Thundercats was un-fucking watchable. But you could look at freeze frames of it and imagine that it might once have not sucked. You can look at a still of Steven Universe and solve that shit by inspection: it's going to be like Family Guy, but with jokes for people who haven't learned not to poop their pants yet.
Once upon a time, American animation belonged to Ralph Bakshi and rotoscopes.
So I get the anger. It's like that time MTV insisted that Kriss Kross was rap.