Why Mercy mains are the most toxic mains - logically concluded
Mercy mains are the most toxic players. That's a logical conclusion. Let me explain:
We have to start by agreeing with the following statement: Mercy is the least directly impactful hero in the game.
In order to get max, or let's say good enough, value from her, you rely on other teammates doing well.
So, ask yourself, if your team cannot get things done with you on Mercy, what options do you have left at that point?
You cannot improve your team's performance much by playing much better yourself. There is barely any room to adapt as a Mercy, as you got little (offensive) utility for yourself. There is only so much you can change by "moving better" or "using a beam more".
Example: A Widow player being blamed for playing her can mitigate the mental pressure by getting kills. Kills feel good for most players, they can serve as justification that you're doing well. They also serve as a way to cope after a loss. You can always cope that you could have done better in a round. That you could have won, if you did something else differently.
Another issue is that besides rez and maybe the "saved by" notification, there is no direct and easy way to signalize to other players that you're having great impact. This is also why especially lower-ranked Mercys might healbot more, because big heal number means big game in their minds. Whenever someone is unhappy with the Mercy, they can say "look at my heals, you just suck and cannot kill anyone". This indirect impact Mercy has is also a reason why others will blame Mercys once the game does not go in their favor.
You can decide to heal or damage more depending on the situation with other supports. You cannot do that nearly as well on Mercy. This is precisely why people like streamers doing climbing challenges on her have to squeeze out any little bit of offensive value Mercy has - to get any proactive competitive edge they can. And where does she have it? Only in her ult. Then, her offensive output becomes good enough for a mechanically good player to get kills with consistently. In a way, the more utility and options you have, in theory at least, the less toxic you tend to be, as you can take things into your own hands.
Mercy is considered one of, if not the, hardest hero to solo-climb with, because you cannot impact the game with your own skill nearly much as you can with other heroes.
So what do you do, if you realize you cannot win with Mercy, while you wanna win? You have to change something, if others don't change something instead. But there isn't much to change in-game for you often enough, as we discussed.
First off: You can always swap to a more fitting hero. Mercy mains often don't wanna though, which is not limited to their mains of course. Or they cannot really play anything else - a super common occurence in Gold and such. You might see the Mercy main swap to Moira and then play her way too passively. In rarer cases, she might swap to Ana, Juno, whatever - but the common denominator will usually be that every other hero of theirs besides Mercy will be played in a healbot style, as they often lack the mechanics to duel.
That's where the inherent toxicity of Mercy players comes in: they have to change things that happen outside the played game itself.
Because a Mercy player is getting value by other players being good, in other words, you rely on others, Mercy players are incentivized to find players to team up with. They will have to look for the right player to pocket. Cue the e-girl and kitten memes - there is some truth to those. And you can explain the existence of these specimen with in-game reasons to some degree.
Now add to this that they will also experience other players blaming them. They will call them out for playing Mercy or just hate the Mercy main being on their team in general. And what do you do if you're being "attacked"?
You will deploy some sort of defense or counter-attack mechanism. Maybe you mute others. Maybe you ignore them and just keep playing. Maybe you throw mean words at those people in response. Maybe you become so tilted over time that you're being mean in advance, as a way of protection. Maybe you start to willingly throw the whole game, because someone dared to hurt you, if your mental is really fickle at this point.
Whatever it is, there is likely going to be a reaction, because the mental of a gamer is more often weak than not. And what happens once things are bad? Once your mood is soured? Exactly, you tend to be more toxic. I would claim that Mercy players are relatively more often involved in such toxic scenarios because of the aforementioned reasons.
I am pretty certain that no other hero in this game has this many reasons why their players are likely to be relatively more toxic.