Subverted: Jack isn't clinically insane, he's just smart enough to look that way to most people.
Double Subverted: There's a thin line between genius and madness, and Jack crosses it a few installments into the story.
Parodied: Adventurers looking to fill party niches always begin by searching insane asylums.
Zig Zagged: Depending on the Author, Jack is insane and skilled, insane and useless, brilliant and eccentric, or just unusually snarky.
Averted: No insane people have special abilities, and no one with abilities beyond the norm are noticeably insane. Crazy Awesome, maybe, but not insane.
Enforced: The story is an Author Tract about equal opportunity badassitude for people diagnosed with mental disorders.
Lampshaded: "Yeah. Figures the nutjob saves our bacon."
Invoked: Bob tries to drive Jack crazier in order to increase his power level.
Exploited: The Big Bad of the setting simply executes any obviously insane heroes he captures- because while a sane man will accept that he's been captured and can't escape, a madman will somehow find a way to escape a literally inescapable prison.
Someone who starts getting a power-inducing mental illness seeks out treatment.
Preserving the powers while minimizing/eliminating the undesirable parts of the insanity is the goal of the treatment.
Discussed: "Seriously? The pyromaniac developed pyrokinesis? Why do only the crazy people get the superpowers?"
Conversed: "You know, the world would be so much more dangerous if insanity came with supernatural powers..."
Deconstructed: Jack's insanity is getting worse, and as it does, his power increases. Cue Face-Heel Turn, A God Am I, or any number of things that can go wrong with immense power with an unstable mind behind it. His teammates are forced to terminate him before he threatens the rest of reality.
Reconstructed: Jack's power, and with it his insanity, can be switched on and off. He may be dangerous while 'in the zone', but can at least be aimed in the right direction.