I'm going to make an ass out of myself and probably have some people call me elitist as hell, but this is why the Mercy main stereotype thing gets around. Name withheld.
This person was /terrible/ for somebody that was in a GM/T500 game with pros about. Like they played Mercy and stood perfectly in the middle of the capture point during fights just casually walking around status. They had to ask before every rez status. GA into Graviton status. There is improvement to be made, but to be made that fast and then show the skills of a plat player? That's a stretch.
This person ended up winning this game btw. Between me and a Pharah going all out we reverse swept on koth to win. This person was very clearly actually boosted by a friend or service using Dynamic Queue, but they can just pick Mercy and pray to god they get carried through matches.
Again, I've played with good Mercy players here before (AJ and Antitrop are both fire), but there are tons of Mercy players at this elo that are hot garbage boosted ass animals who just try and hold onto M1 along with their SR.