Correction, disorders. Disabilities hints a complete lack of function. Disabilities are canon in the show actually, there have been many cases of conditions, especially horse related conditions that have been featured. Pinkie seems to show frequent, off-topic and sporadic behavior in events of the show that get awfully out of context so I wouldn't be surprised at all if she did have ADHD.
As long as no one accounts it as canon or any other related serious behavior there's no harm in speculating further depth into a character, or to maybe make your headcanons and perceptions of a character seem viable and likely.