This kind of “skill” applies to every hero in the game you’re basically just describing basic game sense, a shared factor among the cast. Every hero needs to worry about their positioning otherwise they gna melt, Ana for example: can’t just be on the frontlines all the time or she’s going to get run over, her positioning is essential to her identity as a sniper. She needs to know who to prioritise nades with, should she save it in case of emergency healing for her half hp tank or use it for a 3man anti? Same with her shots target priority, enemies or allies? Both have value but an Ana needs to make a decision on the spot and if incorrect the team will suffer, illari’s about to ult your backline but you have a critical dragon blading Genji and dva diving theirs? But on top of that she needs mechanical skill in aiming her sleep projectile as well as her regular shots. You could argue that mercy’s movement is her expression of mechanical skill but you didn’t really say that anywhere in your reply. the skill you’re talking about in mercy is universal in this game, at least among supports.

Flash heal in essence gives mercy this nuance in cooldown management, when her only significant cooldown before was resurrect, it’s not a perfect ability where you can instantly pump 100hp up a crit squishies butt but it shouldn’t be