I dont think that's right at all. Not in my experience anyway. D.Va's is so much lower to start and it's spread is probably just as bad if not worse than Roadhog's primary fire. Yes she does have altogether further max range... but it does very little damage. If you're within hook distance you're probably losing the DPS battle to Roadhog.... and if you keep firing as D.Va... you can't really move.
And according to this DPS chart
http://us.battle.net/forums/en/overwatch/topic/20744955084
Roadhog has a primary maximum DPS of 225 secondary maximum DPS of 206.25
Diva has a primary maximum DPS of 168 secondary (Bunny light pistol) maximum DPS of 93.25
So I think this analysis is a little too in the vacuum.
First off, D.Va's spread is way, way tighter than Roadhog's. Like, this should be obvious just from playing both characters for a few minutes, and understanding this is actually key to dealing with Roadhog. Outside of point-blank range, Roadhog will just miss most of his pellets.
Roadhog basically has two optimal ranges, one at point blank or very close to it where he can land the majority of his pellets, and one at the explosion point of his canister, where he can do good damage with alt-fire.
In my experience, there is actually a dead zone in between those two areas where Roadhog can't do maximum damage with either attack. He will be missing with a lot of pellets and the canister won't explode. It is a much closer point to Roadhog than you probably feel comfortable -- it's closer than Mei range, but longer than Winston range? I dunno, I wish I had distance markers.
D.Va actually still does close to maximum damage in that range against Roadhog, because her spread is much tighter than Roadhog's. So Roadhog will be doing like half of the optimal DPS above, while D.Va will be doing like 90% of hers. This doesn't include the effect of armor, which is definitely also relevant since Roadhog will be doing half damage the entire time.
But, like, I don't want to theorycraft it forever
I can just say, D.Va is actually one of the characters where I'm personally least worried about Roadhog. I don't necessarily want to stand there and DPS his face, obviously that's dangerous and it's a team game. But I don't think he beats her for free.