Biting your nails is not really considered to be a mental disorder.
In the medical field biting your nails is classified under obsessive-compulsive disorders and not mental disorders.
Anxiety is a form of mental disorder so in a way the nail biting could be a mental disorder for some people because anxiety can lead to nail biting because some people bite their nails when they are having anxiety.
Other people may bite their nails because of boredom, feeling nervous and being lonely or even hungry.
The biting of nails or chewing on nails helps some people relieve stress and tension.
So if you're constantly biting your nails or chewing on your nails it would be a good idea to see mental health professional and get some help and possibly be prescribed some medication to help with your anxiety.
I used to bite my nails as a kid when I was nervous and having anxiety especially during school and eventually I just stopped biting my nails.