As a low level players (I can play Reaper, Reinhart and Lucio semi-decently), I wouldn't be surprised if a Mercy did badly in a game. Her offensive gun is actually kinda awesome if you can hit headshots regularily. It's way better than no-mech D-Va. From my experience, it's like a single Tracer gun without full auto fire. It's just most low/mid level Mercy's forget to use it, the ones who remember it forget to heal properly. High level Mercy is super rare, one who can actually do both supportive offense and healing defense.