NP here. Self-directed humor takes the sting out of a perceived or real injustice.
Of course it's never acceptable for a parent of a "normal" child to make jokes about our disabled kids. Maybe it is similar to ethnic groups joking about their ethnic-rooted personalities/characteristics (never ok for someone outside the group to do so - it would be racist).
Have I unruffled your feathers?
My ADHD kid is definitely more creative and imaginative than yours! In fact, we can never get him to stop, and he hardly knows reality from pretend...