And that means… what exactly? Without context, something like pick rate, is useless.
Btw, what Blizzard actually said, was that Mercy was, by far the most popular hero in the game (I believe at one point accounting for a whopping 33% of the player base). That doesn’t however, say anything about her position in the meta, or her balance, because, as you’ve probably been able to tell through your time on these forums, Mercy is picked for a huge variety of reasons, including, but not limited to, people picking her because the like the character, regardless of how she performs.
If Mercy’s high pick rate (comparable to Reinhardt, who is considered fine, by the way) was also accompanied by an unusually high win rate, then you’d be right about Mercy being a balance issue. As it was, however, and again, like Reinhardt, her pick rate did not also come with an unusually high win rate. So while she may have been picked a lot, she didn’t win all that much, especially in the higher ranks where she was easily countered by pretty much everyone and only excelled in a certain pocketing niche.