I don't think she's making fun of you, since other people are not bullying you as much. I think that she might genuinely be appreciative that you helped her, and be trying to be nice to you.
Obviously, its probably tough to be autistic. I can't imagine. Especially if you are in a school thats not understanding of that.
However if everyone giggles and laughs when you sit down.... then yeah, they're probably making a joke out of it. However, even if they find humor in it, at least you aren't being bullied. You should talk to her in private and tell her that you really appreciate her being nice to you, but that she doesn't have to be- tell her you just did the right thing, you'd help anyone. Tell her that the bullying that happens to you sucks, but you don't need to feel special, just normal really. That might make her realize how you feel and also might help her get other people to back off of you, without her trying to weirdly make it a bigger deal than it is.
Lastly, the hero bit isn't gonna go on forever, obviously... at some point she will get tired of having to bring you over every day at lunch, and sadly, people naturally blame you in that instance so they feel better about stopping talking to you. so its good to talk to her privately before that point happens