No matter which role you play in Overwatch, the ability to hit a target is paramount. It should be no surprise that the higher a player's rank, the better their aim, even if they play mainly support characters.
The good news for average players is, while natural ability and reflexes play a part, aim for the most part is something you can train. Most players in the top skill brackets have been playing FPS games at high levels for years.
Fortunately, there are ways for you to train your aim without having to tank your Quick Play MMR.