I think I know a bit about that. Considering I was probably the one person on the forums suggesting she get nerfed down to 50hp, in the first place.
🕊 Maybe just give Mercy 60hps
In particular, I was using these charts at the time.
Every support's average healing & damage per game for each rank - Album on Imgur
That said, in hindsight that wasn’t so much a Mercy trend, as a Brig/Hanzo trend, which then lead to DoubleSniper and GravDragon metas.