There are plenty of Reasons why Mercy is the most hated hero.
But the main reason is that she is THE ONLY HERO that rewards, with poor mechanics, low skilled Players and Plays and punishes high skilled players and insane plays. Which does not makes sense. Why the ,for example, Zenyatta that manage to win a 1v1 against a Tracer and survive is getting punished by that Mercy who is flying on valk with only one button?