Besides, Q might've been a difficult character for mainstream audiences to take seriously. "Okay, so there's this goofy magic guy who pops in and snaps his fingers and makes anything happen? What is this, the Great Gazoo? I thought this was a space movie, not I Dream of Jeannie!"
Given that we had movies about heaven, time-traveling zombies, the fountain of youth, and a pair (a pair!! I still can't believe it!!) of evil twins, I don't think having Mr Mxyzyptlk in a TNG film would've been a deal-breaker.