I mostly play QP but I'm rising plat in my first competitive season. In QP one of my teammates is silver and trying to improve his D.Va, so I'll tell you what I've noticed with him:

• Not using mobility (horizontal, offensive): D.Va's guns have a steep damage dropoff, so you do much more damage when you're in people's faces. Use your rocket boosters to get close to isolated/low-health enemies and crush them. It's also much easier to hit someone with both rockets and guns when you're close. Make sure you get back to your team before you get punished, this should be easy in Bronze.

• Not using mobility (horizontal, defensive): If you find yourself in a 1v3 situation or similar, and your health is dropping fast, just boost out of there, toward your team and back to safety. D.Va is big and easy to hit, and your defense matrix does not good if you're being surrounded. Find a healer or health pack.

• Not using mobility (vertical): D.Va has some of the best vertical mobility in the game. If you see a DPS hero shooting at your team from a nearby rooftop, boost up there and confront them. It's safer too, D.Va dies when she gets surrounded and ganged up on, and that doesn't happen when it's just the two of you on a rooftop. Even if you don't kill them, it's enough to force them to retreat.

• Wasting Defense Matrix: Don't use it just to eat single Hanzo arrows or whatever, it's a precious resource. The best use is to eat ults, but that's relatively difficult when you're just starting out because most require either quick reactions or can outlast your Matrix. For now, try to focus on high-impact attacks. Moira orbs are good to start with, but so is a Reaper shooting your supports, Rein's Firestrike, or Pharah rockets. If Bastion or a turret is melting you or your team, Matrix him while giving him a full battery of rockets (although this won't take either out on its own)