Well… Mercy is a character that is almost completely separate from the rest when talking about transferred skill. It’s just a “thing” (I can’t remember the word) that she have thanks to her kit.
I you are really good with a hitscan hero, with little to no practice, you are gonna be good with the others.
Same with projectile or beam heroes.
Mercy though, is a character that plays completely different to all the others, except for her pistol and baby Dva’s pistol…
The only thing that Mercy transfers to other heroes is awareness. Which yes, you will need that, but with only awareness, you can’t go past Plat, no mather if that Mercy is GM.