This is about a private property rights issue. Would you think it was ok for BLM protesters to put signs all over your front lawn, because they had strong feelings about the sign’s messages? If you asked them to “please do not do that” 10 times and their reply was always “screw you, we’ll do whatever we want”, would you just say “ok”. All the other talk (tacky/not tacky, etc.) is just sideshow chatter.
Is BLM's dead fiance buried under the lawn?
If you asked them to “please do not do that”
I would not ask them. I would let them grieve in their own way. If I didn't like their display, I would ask the cemetery caretakers to just remove the display in the normal manner they remove stuff people leave on graves. I would not make a big stink about it.
Of course i'm not some pretentious reverend "Doctor", so I don't have the "high and mighty" syndrome.