I don't want to help my disabled friend anymore
Why don't you want to help her any longer?
She acts really stupid
It's understandable to form opinions about people based off their behavior, but its wrong to treat someone differently without any proof of those beliefs. You could end up ruining a friendship over a complete misunderstanding.
It's so hard to be around stupid people all the time.
Ask your friend to stop goofing around and tell them face to face to be more appreciate of your assistance. If they don't see how valuable your time is, then you should try to cut them out of your life.