From an actual support main, FYI:

I just finished tracking my last 120 matches at high plat, having played about 60 hrs of comp this season (I only started tracking because I felt like something weird was going on...). I devoted myself to learning Mercy and getting better/improving and got my win rate to 52% (from 44%). My stats on Oversumo and on Overbuff are Diamond level on Mercy, apparently?? At first I thought, yeah, I just suck, once I master this character, I'll be able to carry with her and I'll see a rise in SR. LOL. I can tell you, though, my average gain is 22 points. My average loss is 25. One game, yesterday, I had 66% kill participation and was awarded 7 SR POINTS! I lost 30 on a game where I healed 52% damage. I watched Eevee, a top 500 streamer, to see if gains and loss averages were the same even for her - and they were. Doing math, I figured out that to play Mercy, you have to win 1 extra game for every 10 you play. It sucks (this was compared using my partner's stats in Masters playing Reaper, his average loss and gain of SR is 22 points, even). I tried Moira too, and it was the same, and even with Ana (I thought maybe it was because she's just so popular, but no). I think it's just a support thing.

That's not to say that you can't improve and go farther. BUT, you are going to have a much slower climb of things than DPS mains. I main Mercy this season, have got nowhere, despite my 52-53% winrate (1 in every 10 games, there's a leaver/thrower lol, and my math is that to rise, I'd have to have a 54% win rate, consistently) and I have literally no idea why anyone plays support when the SR system messes with them like this AND the community treats them like trash. I mean, I even have my Masters partner (who also will fill as Mercy) critique my play every night in VODs and he even says my play has hugely improved. No

I also have NO IDEA how you improve on Mercy outside of playing comp matches... I want to, but man. It's tough. But yeah. I wouldn't play support, tbh.