Healing returns through her healing abilities gives her better effective mitigation than armor. One of the reasons why she has lower than average Tank health and no armor is because that healing that she’s able to get through her abilities is far more effective than any passive armor bonus offered to other Tanks; the only thing is, she has to work for that mitigation.
Everything basically comes down to averages of 30% give or take, that’s usually enough to affect breakpoints, requiring one more shot or even an extra second added to it. For someone like D.va where half her total health is armored health, half of all damage taken by her is mitigated just by her armor. This includes beam damage which tends to be more severely reduced than most mid-range damage weapons.
She cannot immunize herself against beams with Defense Matrix, but she can still out damage all beam weapons used against her, or outlast them.