By estimates, 15% of americans are depressed at some point in their life, that counts as mental illness, that's just depression.

Keep in mind alcohol addiction is also considered a mental illness by some, do i really need to go into how flimsy some definitions of autism is?

At the lowest, by saying "mentally ill people" can't own guns, you essentially take guns away from 20% of the population at the bare minimum, so no, never is this a good idea.

Let's not even get into how everyone on this site would classify as being mentally ill for being a "investigative researcher".