A little lighthearted teasing once in a while doesn't bother me, as long as it seems good-natured enough and the person is normally nice to me. It only bothers me if I'm really obviously struggling. I think it's probably about the same for a person in a wheelchair. It's the difference between "What, you're not going to ride down those stairs?" (absurd and kinda funny) versus "Come on, I could easily hop that curb on my bike" (mocking the struggle).