Hi I'm stopping by to deliver some Mabel headcanons my brain cooked instead of working tha job today so here take them.
She absolutely has ADHD and Autism. She exhibits symptoms of Inattentive ADHD- Experiencing difficulty focusing, particularly in subjects she deems "Boring" (while hyperfocusing and obsessing over those she finds interesting), being generally unorganized and keeping her spaces cluttered, missing important details and having frequent bouts of distraction and daydreams.
Mabel likes to stim by twirling her hair in her fingers. She may also bounce her leg and drum her fingers when seated.
She presently has no idea that her usual "Mabel-isms" are anything out of the norm. As a child, both she and her parents either don't notice the signs or are in denial of them and so she doesn't get much in the way of treatment. Mabel would get herself diagnosed only as an adult.