For some reason people are stuck on "overhealing bad" not understanding having high overhealing is only a sign of being bad if you have low actual healing, the worst healers i've ever had to deal with in this game playing as a healer from vanilla->now are the ones that try to actively avoid overhealing for whatever reason.
Theres 2 main situations where it's worth to look at and consider overhealing as a problem.
The obvious one, Healer goes oom early, casting expensive heals when it's not needed may be the reason, indicated by large overhealing with expensive spells.
People keep dying when there was plenty of time to heal them yet they weren't healed, one or multiple healers may just have their cursor on the raid frames spamming aoe healing instead of actually being ready to spot heal spike damage, or just trying to pad meters, not directly indicated by overhealing but it's a likely sign.