For D.Va, one of the things that has helped me recently is getting better control over your "bunny hop". Tapping shift while flying cuts your movement short, allowing you to do tiny hops, which are SUPER helpful.
D.Va's main counters are Tracer and Roadhog. Roadhog, because he disrupts her completely, pulls her apart from her team and singles her out for your enemies to target you all at once. Tracer, because she can zip around D.Va, harassing her while avoiding her defense matrix, rockets, and primary fire.
D.Va plays almost like a DPS. But you've gotta be careful not to overextend your role. She's mainly for disruption, (eating ults, booping enemy teammates, being a bothersome and hard-to-kill target in general), so make sure you're focusing on what she's best at. She also serves to protect supports from flankers. She shouldn't be diving in very far beyond her team. But she is in and out. Just not as much as, say, Genji.
I hope this helps! I'm not a very experienced authority, (only gold ranking), but I've watched the league very carefully and I do feel I have a pretty decent battle sense.
