Other than the raining frogs bit. If he cured the inmates no-one would notice, ever heard the words "I'am not crazy" out of a crazy person's mouth? As for the doctors, should he the son of god do something as evil as makeing a sane man insane, they would just lock up the docs saying it was stress form work that did it.
Mental hospitals are a form of Hell to the patients, I'm pretty sure that cureing them wouldn't be a bad thing. And how do you people know He isn't going to come back soon? He could come back at any time. (God = He = Jesus)