After he had the power of god? Many of us would stop caring at that point, too.
Before then? "Remember those rodent things on Dimorus, the poison darts they threw, I took one meant for you." "And almost died." The real Gary loved Jim Kirk, had a cordial relationship with Spock and was a bit of a womanizer--I don't see him as a jerk. I see him as the kind of friend many of us would love to have.
I disagree. A good parallel to Mitchell's situation was that of Cmdr. Riker when he was granted the powers of the Q. Those powers were even more god like and it didn't even take time to ramp up. Riker's first response was not to go on a power trip, but to try and help his friends.
Mitchell's first response was to subtlely threaten Kirk in their first meeting after his accident. Indeed, you can't help but notice that he never once offers to help fix the ship, which was severly damaged.