No, it is not "toxic" to play a support character, spend every second of every match either healing or rezzing, and literally never saying anything bad to anyone else.
You both cheapen the word "toxic" when you say that.
And frankly, you don't get to control the best character for everyone to play, just like I can't control when you play like shit.
It is absolutely toxic to refuse to work with your team when something you're doing isnt working out. Like I said, though, Mercy skirts this issue by being a pretty solid character on the ladder, and doesn't really have a hard counter. There was a time, though, where Mercy was not great in comp 100% of the time and if you refused to swap to Ana or Lucio in bad situations then you were making it harder on your team than it needed to be.
Again, There are characters in this game with hard counters and when you're getting hard countered you should swap, or talk about a swap with your team.
I want to stress here that this isn't a case of me blaming my teams for losses. I know that's an ego problem in comp, and it's toxic as fuck as well. I just don't think people put enough thought into character swapping in a game where it is often necessary.
If youre using battle Mercy you aren't doing your job 99% of the time. And yes, she is a low skill hero, and theres nothing wrong with that. She just also happens to be a low skill hero who is incredibly efficient at support and ostensibly can be used to get around SR losses/gains.
I'd call Mercy a low TECHNICAL skill hero. Continuing to call her a low skill hero isn't doing anyone any favors here. The skills that a person who is going to main mercy need to develop are more game sense related than aim related. You need to use your far backline position to your advantage and make calls, sometimes for people.
Example:
"Hey can someone help our soldier, he's got a genji jumping all around him"
"Hey I see a reaper approaching from the left, he's probably gonna try to flank around the building, let's pull back and take care of that"
"hey, I have res, so their mercy probably also has res. We need to take her out first"
"It's been a while since we've heard graviton surge or missile barrage, let's be on the lookout for that, dva try to keep your defense matrix in Zarya's face"
Stuff like that. And if youre a mercy main who isn't doing this or isn't willing to do this then you aren't playing the character to it's full potential and you should work on that.
Is it toxic to play random characters even when they are not optimal for the type of game you're on? Got a bunch of hate messages for playing Sombra, Orisa, or Torbjorn. I just like playing random characters for fun with my friends.
Only if you're in comp and it's not working out. People catch flack at the beginning of games for locking in on Sombra or Torbjorn and that's fucking ridiculous. I'll always give people a chance on their shit before even suggesting we rethink our team comp (unless we start out with no tanks or healers or something).
Obviously no one should care what you do in any of the other game modes.