Is teeth grinding a mental disorder?

Teeth grinding. Chronic teeth grinding and/or clenching (bruxism) is not a mental disorder. It is a habit, usually in response to stress. It may also result from an uneven bite. Bruxism most often occurs during sleep, but also often occurs during waking hours. It is also common in people who have sleep apnea. Talk to your dentist about behavior modification during the day and treatment for nighttime bruxism.
Definitely. Someone may grind their teeth for a number of reasons. Often, the problem occurs because the muscles of the face and jaw are trying to get the teeth in a comfortable position. The muscles may be in spasm, you may have a tmj/tmd problem, stress, infection. Best to see a dentist asap. Often thew cause is difficult to determine, so may have to treat the symptoms with a mouthpiece.
I hope not. I have had periods in my life when i found myself grinding my teeth. No, bruxism is not considered a mental disorder. People with mental disorders might grind their teeth, but the reverse is not a valid corollary.