Yeah, a strong magnet near an LCD screen will cause weird colors. I wouldn't worry about a magnet being used to hold a dashboard mount in place, though. You can always give it a shot and stop using it if you do see any ill effects. I don't think you can permanently damage the screen with brief exposure to the magnet, and I'm almost certain that you'll be able to immediately see if there's a problem. There won't be any non-visible damage that builds up over time.

You may also want to consider a suction-cup mount. Usually these have a smooth plastic disk which attaches to the dashboard with foam tape, for use where windshield mounts are undesirable (or illegal).