If Mercy is using the Caduceus Blaster, it means that she isn’t healing or damage-boosting. Her blaster is just a last-resort weapon. If she’s being jumped on, she will likely die without her team’s protection, unlike the other support heroes, who are more independent.
Damage-boosting isn’t the same as dealing damage yourself. Mercy can’t use damage boost to solo-kill enemies or defend herself against flankers.
Valkyrie lets her fly, but it doesn’t exactly do damage. She can get close and use her blaster, but again, she’s not healing or damage-boosting, and it’s risky flying so low and so close to the enemy.
Mercy’s passive, Regeneration, is Mercy’s way to stay alive. It’s a survivability passive. It only takes effect after Mercy has been out of combat after one second (i.e., not dealing or taking any damage for one second), meaning that it’s useless in a fight.
Moira, Zenyatta, and Brigitte are damage hero hybrids. All three can heal while dealing damage at the same time.
Lúcio can heal while doing everything else, like shooting at enemies, and he can heal multiple enemies at once.
Ana can heal multiple enemies at once with her Biotic Grenade and has three different ways of healing (Biotic Rifle, Biotic Grenade, and Nano Boost). She has extremely high burst healing capabilities, while Mercy has no burst healing.
I’m not saying that Mercy needs high-burst healing, but her healing needs to be increased back to 60 health per second (or, at the very least, 55).