The overall win rates are just a small part of the story. Her win rate gets really bad when you go to high ranks. At those ranks, it’s much lower than the average win rate, which means that playing Mercy will cause you to derank. None of her competitors is even close to that position in high ranks. She’s in a very bad spot there.
I wrote a post containing the statistics about this a few days ago.
For those of you who feel Overbuff statistics are unreliable - that’s just not true. Here’s a post I wrote with a detailed explanation of how to understand them in the age of private profiles. I believe it may have the distinction of being the only forum post based almost entirely on math to have been the front page.