As D.va, the secret is to make them care about you. Boost in, attack. Boost out, above; find medpack and heal. Boost in, kill. Defense matrix as appropriate. Stay just out of their reach while alone, or stand proud in a group; the enemy will struggle to kill her either way. If mech is downed, either go out in a blaze of glory, or retreat and pepper them with the peashooter until it's back. Once her ult is ready, figure out an enemy holdpoint, boost the mech in that direction, and let the self-destruct do its work.
She's not great at surviving sustained attacks like Reinhardt, but she can be incredibly tenacious if she boosts in and out of battle. Her maneuverability is second to none thanks to the boosters, and her basic attack is literally relentless. Coupled with a defense matrix that can give decisive protection to D.va in an approach, or allies attacking from behind her, it makes her an incredibly dangerous threat. And then, of course, there is her room-clearing ult, perfect to bust any enemy bunkers. This is without getting into her almost sniper-esque form when out of the mech... she can't do much, but her damage output can be surprisingly consistent.
In other worlds, D.va's basically the bomb.