"Not playing your best" is not throwing.

TL;DR It's OK to play a hero to learn it and not doing everything you can (including the hero pick) to win the match.

Playing a hero I'm not really good with in competitive is not throwing the match. I'm a tank main and today I picked Mercy (because we needed a support and I'd like to learn how to play Mercy in competitive). Although I tried to play up to my best, I realize my positioning and game awareness was terrible (Mercy is not a tank) and that I, more than everything else, was the reason we lost. If I picked, for example, Zenyatta or simply a tank and let someone else heal, we would have much higher chances to win the match.

Playing in order to learn a certain hero/strategy/combo/... instead of doing what you know best and tryharding to win is IMHO alright.

I would really like to learn to play properly a whole bunch of other heroes (I suck as basically any DPS), but there's a lot of pressure from the community (i.e. the team) to make me play my mains because everyone just wants to win every single competitive match they play.

Whenever you encounter a player who is not playing really well or who picks a hero that's not his main, don't talk him out of it, please. Is he lowering your chances to win? Yes, definitely. Will you lose the match because of that? Maybe, probably, maybe not. Are you making it a better game by allowing him to play what he wants, letting him learn other game-style etc.? Yes, yes you are. Please do so.

And just to clarify, I don't mean playing a hero in competitive with zero playtime in QP. I've got more than 2 hours of playtime in QP on every hero except Sombra and Doomfist and more than 6 hours on a majority of them. I'm talking about learning the advanced stuff; the positioning, the awareness, correct ult management etc. The stuff that just can't be learned in QP.