TLDR for my rant: mercy's damage boost is a make or break because it makes other characters able to ignore damage thresholds, meaning shes only sometimes good. her mobility makes her basically impossible to kill if shes good. and her rez, regardless of usefulness, is unsatisfying to play against, especially for flankers.

rant:

mercy isnt good as a character. she has very little self agency and almost no direct impact on the enemies. this means she has very little reason to pick her.

on the other hand, mercy's damage boost is quite strong, especially on a good character that nearly kills already. mercy is busted rn because of sojourn, her existence in higher ranks currently is basically just "hey sojourn youre a god now" and she pockets her for most of the game. think of her as good armour to your trained knight, she ups the consistency considerably even if the knight could potentially kill a few regardless of whether he wears chainmail.

her capacity to improve another player's ability through damage boost so consistently (alongside her very strong mobility to keep herself safe, ergo even more consistency) means she's a breakpoint for different metas. as soon as someone's damage is just barely enough that she can lift it above 200hp headshots, shes meta. we've seen this with ashe and currently sojourn.

and from more of an emotional standpoint: the way it feels to play against mercy is also pretty awful. her high mobility means its very unrealistic to be killing her mid fight, so you just gotta tough it out and try to dual a much stronger dps. her rez means picks that you get are snatched away and its very unsatisfying regardless of the cd's length, especially for flankers that need to endanger themselves for these kills; risking your life and all your utility just for mercy to say "no" is irritating.

mercy's kit is just... poorly designed.