There are better ways to say you don't like something/how someone was portrayed or adapted and not be rude about it.

I don't think it's wrong to comment on an actor's appearance when discussing casting. Looks are literally part of the job.

If I think an actor is unattractive, I'll say that. If I think their face is too chubby for the character, I'll say that too. And if a costume makes it look like they don't even have a neck, I'm going to point that out as well.

None of that is "body shaming" to me. It's my opinion on whether someone is physically convincing in a role. You're free to disagree, but I'm not going to pretend appearance suddenly becomes off-limits just because we're talking about a real actor.

As for the "being nasty" part... I don't really care if you think my opinion sounds rude. I'm not trying to win points for being nice, and I'm not interested in living up to someone else's idea of what a "good fan" is supposed to be.