His D.va is better than average. His Mercy is pretty solid.He plays D.VA pretty average and plays Mercy pretty good. Don't be dumb, if you can't swap to any hero that is needed then you can't be considered good. I'm not saying you people are bad, I'm saying if you refuse to play a majority of the cast you can't say shit.
I play Mei and only Mei well, and that doesnt work sometimes. Hence why I'm the worst.
However, D.va and Mercy together cover nearly any situation so he doesnt need to know the whole cast.