While Mercy typically has above a 50% win rate, so have plenty of supports that got buffed anyway.
I will be using GM stats from a couple months ago for the following argument, so it may be slightly outdated:
She’s very consistently the second lowest picked support (or lowest) and has the lowest healing of all the main supports, reaching up to 3k lower. She only beats Lucio and Zen. She is on par with Brig, who also provides 5k armor on average. Sure, she has resurrect, which averages 6 per match. For consistency sake, let’s say she resurrects 2 tanks, 2 DPS, and 2 supports on average. This means she is “healing” 1,800 additional healing (not entirely accurate. I just used 500 HP as tank baseline), meaning Brig is still doing more overall, leaving her as the 3rd lowest.
She is apparently supposed to heal the most over the course of a match, but only keeps up with off-supports in healing output.
Her average damage amp the last I checked is 1.1k and 500 damage done. 1.6k total. The SECOND lowest damage dealer (Ana) does over 4k per match on average (can’t remember the exact number), which last time was over 3x more than Mercy’s damage AND boost combined. The SECOND lowest. Also, the second lowest eliminations on average (Ana) was 12 elims on average. Mercy’s damage boost assists and elims combined was 11, meaning both of her stats combined was STILL beaten by the second lowest average elims support.
Then again, her healing is the 3rd lowest.
She does nothing better than any of the main supports except resurrect. Her damage boost isn’t worth it, because with her damage and boost combined, it still doesn’t even equal to the second lowest support, even if it is “trash” damage, that trash damage still averages more elims.
She needs ONE thing she does better than the other supports (stats-wise), but she doesn’t. All of them are worse. She definitely needs help somehow.