Nah it was a smart move it think.
Nerfing mercy’s HPS doesn’t mess with the feel or flow of the hero like messing with her GA cooldown or rez has the potential to. All it does is nerf her healing output a little bit.
Couple that with the buffs the other supports got they should be much closer to mercy in overall healing. This is a good thing becasue it will go a long way towards diversifying the support meta, which is needed so very badly.
Accomplishing the above without mucking about with the flow and play style of a hero is a good way to approach balancing. Otherwise you end up with another sombra