Personally, I don't think THC is good for those with mental disorders. As others have pointed out, it could possibly help in the short term, until it becomes problematic. Those with mental issues may be more likely to abuse it.

That said, CBD could be helpful for those suffering from anxiety or sleep disorders. You don't get any kind of high from CBD, just relaxation, so it's potential for abuse is far less.

I will point out that I abused marijuana. I have bipolar disorder, and it calmed everything down. Problem was, I craved it badly, and eventually was stoned for a good part of every day. It ran my life. Been off of it for 4 years now, and will never touch it again.