Well, the first reason is one you mentioned yourself; he's bipedal and intimidating. Every single Fire starter final evo is bipedal and intimidating (I guess you could maybe make a case for Typhlosion being quadrupedal or Delphox not being that intimidating, but you've still got 6/7 starters for both adjectives). People are getting really sick of the same thing, which is why Incineroar is getting so much hate. Plus, it could very easily pass for a dreaded Fire/Fighting final evo that people hate so much for the same reason as before, despite it not being one.
The second reason is this: look at Litten. What do you expect from it? Probably Fire/Dark, but it has witch sock like markings, it looks skinny, which typically implies weak physical stats and a bored, suspicious face, which implies that it will be a special attacker that looks fairly weak physical-wise. And like I said before, people hate the bipedal, intimidating, Fire/Fighting design. So what do you think the reaction is when that's pretty much exactly what they get? They'll think the Litten design would have been wasted for more of the same, so of course they hate on it. Their expectations were let down.
Anyway, hope I could help! If I'm missing any other important reasons, let me know in the comments.