If you're in Quick Play, play whatever you want. It's the only way to learn, so people should just deal with it (as long as you're trying). Although if there's already an Ana and a Lucio, you don't really *need* to go Mercy. The team will probably benefit better from another tank, or whatever.
Some heroes get picked on more than others, because either they're harder to master, or they just don't fit in certain maps. Hanzo and Widow gets picked on a lot, especially Widow since she's not viable on most maps (unless you're very good at her).
I wouldn't say people hate Mercy. She's super easy to play, and does great healing (although Ana is better, but that doesn't matter). Anyway this is all a whole different story if you're playing competitive. But if you like the game and wanna keep enjoying it, stay away from competitive. It's way too toxic. lol
Again, if it's Quick Play, play whichever you want tbh. But it's good to learn 2 of each role so you're flexible enough to fill roles or switch if you feel you're doing bad. I'm trying to play Widow more but it's hard because she won't work with a lot of maps. So I often switch to DPS or tank if we need more of a push. D.Va is always good, and works for all maps imo. As long as you're having fun~
