It's kind of a taboo subject and no one really wants to talk about it, but I feel like there needs to be a discussion about it.
I don't think League causes mental health problems, but I find from a lot of people I know and personal experience, that a lot of people who do have them use League as a sort of escape. The way the game is structured, it is easy to queue for game after game and kind of just get away from your problems in your mind/real life.
It'd be nice if somewhere on the website/client Riot addressed mental health somewhere. I'm imagining something where they encourage people to consider seeking help in real life if they need it, reaching out to friends/family/school counselor, and taking breaks from gaming at least every few hours.