basically like any character in the game? ;P
No. No not at all.
You don't need to aim well if you're playing reaper, you just need to know how to close the distance. You don't need to aim if you're playing Winston, you need to know how to move through the battlefield. Junkrats get half their kills spamming grenades in the general direction of the battlefield, Symmetra's gun tracks the enemy, Roadhog blasts enemies faces that get close, Torbjorn's turret does the aiming for you, etc. You can do well with all these characters even if you have complete shit aiming, because Overwatch is a game where knowing the maps, the characters, and communicate with your team will give you far better results than mere good aiming. Aiming will help with all these characters too, of course, but you can have piss poor aim and still have plenty of viable options in which to contribute to the fight.
Then you have characters that ARE more aim dependent like Soldier, McCree, Genji and so on. But 1. they are both mobile and in far closer range, so even with them, aiming is only so difficult compared to the pixel hunting Widow has to do for headshots and 2. more importantly, they all have abilities that allow them to compensate for poor aiming. Soldier's ult is to become an Aimbot, Same with McCree and he can Fan the Hammer on a stunned enemy too, Genji can dash and has the dragon sword, none of which require aiming to be useful. They can be difficult to pull off because of positioning and circumstance, and you need to improve aiming to be useful consistently with these characters, but you can be shit at aiming and still contribute due to these abilities.
The second most aim-intensive character in the game is probably Hanzo, and even he has scattershot and his ult that can do work without good aiming. Ana too, but she has an easy time hitting her allies and has her grenade. Pharah you can argue is another, but her rockets have a generous range to them. It's not hard to spam them and get results. Plus, occasionally, she can people off ledges with her concussion blast.
Widowmaker is the
only character in the game whose entire utility is based on being precise with her gun to a degree no other character is held to while also having nothing else to back her up in terms of abilities.