It’s an interesting dilemma to be sure. Stuttering can indeed be considered a disability but many people who stutter don’t consider themselves disabled. Because the ADA definition of disability is fairly broad, and you can be considered to have a disability if you’re regarded as such, I think it’s probably best to not check it on the application form. BUT consider disclosing at the start of your interview that you stutter so it’s not the “elephant in the room.” I was fired because of stuttering at work so this issue is very near and dear to me. Good luck!