It depends on the situation. There are certain KotH points where this is brutally effective. And again, this is a teamfight, not a solo D.Va situation. I don't even use D.Va more than one game in thirty, and I've gotten quads post-patch by shrugging and tapping Q when a fight's going badly.
Every other powerful offensive ult in the game, save Hanzo's (lol), requires you to put yourself in harm's way or risk being shut down. D.Va's is unstoppable, has zero risk factor, and then she gets to hop right back in her mech and keep trucking.