I’ve literally seen battle mercy players who have plenty of kills and damage on top of healing.
Hell, I’ve played mercy as a ‘battle support’ character, swapping to my pistol and firing off shots when there’s no targets around to heal or provide a damage boost to. And her damage spikes when using her ult, during which I’ve seen multiple mercy players get quite a few kills playing that way.
You will get out of Mercy what you’re willing to invest in her. If you’re only going to hang around and damage boost/healing stream all the time, then yeah, you might find that the game is boring and your character sucks, especially if the person you were healing/damage boosting dies and you end up exposed. But if you’re willing to swap to your pistol actively and deal some extra damage between moments when you need to heal, the game is a lot more fun.