I don't really have a take here, but the other day I was watching suicide squad and the big shark dude's entire character boils down to him being strong but intellectually disabled. Is it ok because he's a shark? The underlying joke is still that mental disability is funny. What about big dumb human henchmen in films and cartoons? You know the ones, who go 'uhhhh boss what are we gonna do?' in a 'slow' voice and the boss is like 'get them you imbeciles'...
In the moment the audience and I found it funny, but I never really thought as to why.
At what point does someone go from being stupid to mentally disabled? Why's it ok to make fun of someone for being stupid but not mentally disabled? Neither person can help their mental ability.
Like, I've got a couple friends who are perfectly functional but at school no matter how hard they tried they'd never get good grades. Sometimes they'll say some dumb shit, and people will laugh at them for it.
Stupidity in fiction is pretty interesting when compared to stupidity in real life. Stupidity is such a large part of the human experience, literally everyone is sometimes a little slow on the uptake, doesn't think enough before acting, believes something that is obviously false, has bad logic, etc. In real life, humor mined from this is often completely harmless.
But in fiction, there are characters that are smart and never dumb, or dumb and very rarely smart. If a scientist, who are often supposed to be smart in fiction (contrary to real life), acts stupidly, it is often seen as a mistake by the writer. If a stupid character acts smartly it is seen as a subversion of a trope. This in effect essentializes smartness and stupidity, and dehumanizes the characters. So all dumb characters become essentially dumb and dehumanized, which has unfortunate parallels with how mental disability is seen even when that was not the intention.