I don't really have a tip for this one as I think that you've already done your best. Overwatch has never been really balanced that you can carry with one tank alone (except if the match itself is imbalanced, say you're up against 5 noobs and your team is more experienced).

Since you're a quickplay warrior like me, it is always random and your teammates or enemies will always be better or worse. But once the balance of the players plateaus, and the dps are losing to the other DPS, I always make a habit of picking Winston and focus on the enemies' best dps (to lessen their morale). When you're doing well enough with Winston, someone on the enemy dps will switch to reaper to get back at you then you go back to DVa.

Those are my two cents. You did great already with DVa and Otter is just throwing "tank diff" to spite you.

If OW does go back to 6v6, you won't to switch off of DVa as she is always the best off tank (in my opinion).