The problem is people hate the fandom as a whole. It's exactly the same as generalising an entire race - saying all chinese people love noodles, or something like that (I don't know, I'm bad at examples). It's plain wrong to do. It's also the fact that a lot of people don't even have legitimate reasons to hate on the fandom, they just do it because we like something that they think we shouldn't like, and again, that's not okay.
Criticising certain elements of the fandom, like the rule 34 making part of the fandom, or just certain people (AND STATING THAT THEY ARE ONLY TALKING ABOUT THOSE CERTAIN PEOPLE), is fine. Criticising certain elements of the show, again, is totally fine - but saying the entire show sucks without reasoning isn't.
It basically comes down to people making blanket statements about the show and its fandom. I hate that and I think most reasonable people hate it too.