I’ve been thinking about this question since both communists and vegans are well known for being demonized by capitalist countries, and so I thought “maybe PETA might also be demonized since it’s an animal rights organization”. Of course, all I know about PETA is that they make parody games, and that everyone seems to dislike them, so feel free to prove that I am in fact stupid.
No, they’re not as bad as everyone says. They’re far from a perfect organization and I think some of their ad campaigns are questionable, but most of the hate they get is, exactly as you suppose, carnists demonizing PETA for being staunchly pro-animal rights.