Her damage is not low at all and she's a fantastic flanker-- she's not supposed to be shooting from a distance because you'd just be giving your position away. Take side routes on the map and zip up behind people all close-and-personal-like and turn them into swiss cheese. At point blank, she can drop pretty much anyone before they can drop her on a 1v1 basis. When you get ganged up on, yeah, you're probably going to have a blowout, but hopefully you prepared self-destruct before putting yourself in that situation. Either way, if you're not practically hugging them while you blast them, you're not using her at full effect.
My MO with D.Va is to lay low and ambush, and then immediately have an escape route planned toward a med pack if things go sour. I typically try to pick off opponents who are distracted by my teammates ahead of them (Reinhardt is the most hilarious target because he's a complete sitting duck from behind when he has his shields up.) If need be, I'll start boosting forward and self-destruct, effectively turning her mech into a gigantic aimed nuke, which also gives me some distance from it.
Also, she has a huge advantage on maps like Volskaya where the path to the second objective is so damned narrow. D.Va can simply fly around from the right onto the scaffolding and hop on down right into the objective, which allows me to pick off low HP targets and annoying Symmetra turrets before I die or self-destruct.
I guess that's what I really like about her-- she's a defense-based unit meant to be played very aggressively. She's highly mobile with a ton of HP, and deadly at close range. At best, you can clear a huge cluster of opponents. At worst, you can be a distracting high HP target and allow teammates to pick off distracted opponents.
