"Excess frosting" refers to having more icing or sweet topping on a cake or dessert than necessary. It suggests there is too much of it, which can make the dessert overly sweet or heavy. Full definition
I made these cupcakes this week and shared them with my dissertation group and sent some to my husband's work (along with a tub of excess frosting). (pinchmysalt.com)
Place the first layer on a cardboard cake round set on a rotating cake stand, with strips of parchment placed partially underneath the cake on all sides to protect the cake board from excess frosting. (brighteyedbaker.com)
This way, any excess frosting can be smoothed around the sides of the cake when you place the top layer on. (epicurious.com)