I think what upset me about the "joke", very precisely, is that is is aimed at all those medieval, atavistic views about those with mental disabilities: that they are dangerous, hypersexualised, a danger to normal folk. It is attitudes like this which led to abuse of the disabled through history, and which still contribute to abuse, bullying and even murder today. It dehumanises the disabled, meaning normal rules don't apply and people feel free to throw stones or abuse. It is happening on buses every day - "retard" "mong".
I also find it cowardly - that right-on comics like FB will throw massive flounces if accused of racism, even suing a paper. But the disabled? Fair game, as still not governed by the pc rules. Is it cos a 9 year old autistic boy can't harm FB, but a gang of grown men of different races could. Can't stand cowards and hypocrites.