@JenJenJenJen - my D has ADHD, and she made it a theme of her application… At least she wrote a whole essay about it for the “anything else” question, and she told her recommenders they were free to talk about it.
My personal opinion is that there isn’t a stigma with ADHD. I think if anything, it helped my daughter on her apps, because it helps explain her grade (trend).
And to be honest - if a school penalizes her app because she has ADHD - that seems like it’s a school that is going to be less accommodating anyway, and you probably don’t want to go there.