Yeah I guess so, I actually have never explored this as a theory, of why some people think it's funny for someone to be sick or disabled. I've always thought it was mean, even when I was a kid, though I sometimes said rude honest things as a small child, like that my grandmother smelled bad when she was dying of cancer, I didn't necessarily find it funny.
People tend to laugh at things that embarrass them or shock them, like have you ever laughed when someone randomly hit their head really hard? And you know you'd be pissed if someone did that to you, but it's like an instinctive nervous laugh or something?
I have this sort of sense of humor where I do laugh at things that are physically incongruent (like a four year old in her mom's sexy panties, because four year olds aren't sexy and the panties don't even fit her, it actually made her step-father angry that me and the girl's mom automatically thought it was funny that her four year old did this) ...but I don't think it's funny for people to be disabled, mentally or physically.
I do laugh at mean things, though, like my verbal humor can be very "mean" and some people would wonder why I think those things are funny...and I don't know. I just remember being 9 years old and already thinking a black comedy like Clue was hilarious, and then at 15 laughing at Uncle Vanya. Some people don't get that either, how cruelty or misery can be funny.
So I'm not perfect. I just don't like to make fun of people for things they cannot help, and I think it's childish or cruel to do so.
Maybe it's a form of insecurity? Like I'm the big, normal, healthy person and I'm picking on you because you're safely not? I've noticed men do it more than women, or maybe that's my imagination.