I think Nowchowederforyou's got the right idea. I don't think its wrong at all to have a laugh with a mentally disabled person, as long as you've swallowed your pride enough to 'switch gears' and get somewhere near their level. This sounds bad, but its not. I really think that words like 'better' and 'worse' get overused in terms of different mentalities, especially with autism. It's not a "lower" or "worse" level of being, just a different one.

I have an autistic cousin and we have an absolute riot together. It took me a while to realize how genuinely funny he is - when I was younger I would shelter myself from any possible embarassment by talking to him like he was something less than a fully-realized human being, like he didn't have feelings or internal reactions to anything I said to him. It was a prickish way to treat a person, but I think its not uncommon for people who aren't used to dealing with 'mental' people - autistic people, anyway - who are often very coherent.