But why dead last? Would the failings you mentioned not negatively effect loads of other heroes too? Is Mercy really so difficult to play that poor use puts her behind every other hero?
I dunno, it seems weird to me. Like there's something more complex going on.
Mercy is to Overwatch as Io is to Dota 2. Io has been one of the strongest heroes in the game based on his competitive presence. He's easy to kill, and all he does is buff other heroes, making them attack faster, move faster, and heal faster. he can teleport a hero to any place on the map with his ultimate ability. now find io on this pub winrate list
Lucio however is the Omniknight of Overwatch. omniknight has a heal that also does pure damage in an aoe, a skill that makes a teammate magic immune, and a skill that makes everyone of his teammates that are near him immune to physical damage. super underpicked in competitive. look at that list again and find omniknight's winrate
Mercy takes too much coordination and communication for most pubs to do well with her.
its also that Mercy is waifu bait, so of course waifu nerds just jump to her when they need a healer without knowing how to play her instead of picking lucio and just turning the heal on