My 2 cents:
Mental health impacts all specialties and all doctors will encounter it in some way, not just psychiatry. It's good to have a central theme to your personal statement, especially if that subject is what really interests you and is what drew you to medicine. It depends on how you handle it. It's fine to focus on mental health, but you don't want to come across as "omg psychiatry or bust!" You can certainly have a specialty in mind right now, but you should be open to other possibilities you may not have considered. Have some trusted people read over your PS when you finish to make sure you don't have blinders on for psychiatry, but otherwise I think you should be fine.