Good aiming is, of course, a must on shooters, that’s a fact, however, Overwatch is a competitive game and in almost every (or every) shooter, good aiming is not everything, but it surely helps, the reaction time and the accuracy are crucial factors in some situations, but even with that, people with good aiming, can be in lower tiers than others with worst aim.
The reason for this is pretty simple: There are many strategies to follow, path to take, comps to do and, of course, in Overwatch not everything is about aiming. There are heroes with autoaim, such as Moira, or melee heroes, such as Brigitte or Reinhard. Even if we think about aiming as a priority thing, we can’t forget the heroes size, shields size and other factors. Every tank is quite big and some DPS have a big hitbox as well, it’s quite easy to hit at least something. Landing every shot or landing shots to a Mercy/Pharah, it’s a different thing.
For getting higher tiers everyone need to understand their role and the heroes they are using, healers need to heal, tanks to tank or even, engage, DPS do damage or hunt Mercy. But there are ways to do it, you can’t simply use the shield and hope that’s enough, you need to know when to stop using it for recharging, or know where to step to make it worth using. Those are things not every OW player knows, of course high tier players knows how to do it and you can play with some of them sometimes to learn and most likely, win often.
Also learning about the maps and the paths you can get into the zone with your team is well needed, again, you can’t simply go between them by your own as much silver players do, coordination is needed, and if there is no coordination, probably there isn’t in the enemy team either and you can use it as an advantage, for example killing the only worth-player in the other team or doing tricky movements to juke them. Also, good engages with the enemy dispositions. Those are opportunities you need to make or identify to take advantage of them. If you can do it, then you’ll climb in no time.