Is D.Va's ultimate affected by Mercy's damage boost?

Short answer: Yes

Long answer: I tried in a custom game with regular damage and health settings, and my results were simple:

• At the center of the explosion, Self-Destruct will one-shot any unbuffed player character, regardless of health, which means it deals 1000 damage or more.

• At a few meters, I managed to have a Roadhog survive the explosion with a little over 100 health, which means the damage has a fall-off based on the distance from the center of the explosion.

• At the same distance, with Mercy's damage buff, the same Roadhog died to the explosion (I may have positioned it a little off the original position, as I didn't use a ground mark - in hindsight, writing this now, I should have, and maybe will test again later).

• I had Mercy's beam fall off one try (because I think it normally gets reapplied instantly to pilot-D.va when she comes out of the meka) and the damage boost didn't apply, and Roadhog survived. Which means the damage boost is applied at the moment of the explosion, not of the eject. So only if Mercy is buffing D.va when the mech explodes, the damage boost is applied.

I took liberty and tried with a couple more "external-based" ultimates, and came to conclusion that it works with most of them, but not with Junkrat's Rip-Tire for some reason, which is probably a bug that should be reported.