I'm a low Gold, maybe high Silver, D.Va player so take what I say with a pinch of salt, it probably doesn't translate to the higher level play...
... don't get in people's faces all the time. It doesn't feel right at first, but I basically spend my time either behind the Rein helping to soak up the damage his shield is taking (block enemy Rein firestrikes, block explosives, etc) and then chasing away the enemy when the focus is less on them firing back at you.
Without a Rein, even sometimes with, I'm usually on the backline. I use DM against tracers and reapers that are trying to mess up our supports while being close enough that while they're on reload putting some hurt on them up close.
I'll sometimes boost out to take on specific targets... Widows, hurting winstons, Pharahs if they're not good at keeping up high... but mostly I'm opportunistic in how much I get up close, and when I am it's more often than not to use DM to protect people who are on a push.
It's only very rarely that the team I'm on is good enough that I can go full on disruptor and just fly around being chaotic and breaking the enemy's unity, but even then it's not about doing damage, it's about diverting their focus.
Without a doubt the primary reason I feel I'm on the field as D.Va is to help stop others getting ripped up.. either supports from tracer/reaper, or everyone else by using DM to block reaper/soldier/pharah/roadhog/bastion/Zarya (if I'm on the ball) ults. Killing things is not why I'm there, unless I can work a good ult in