While it is possible to ice a warm cake with caramel icing, it is best to let the cake cool completely before icing. Icing a warm cake may result in the frosting becoming contaminated with cake crumbs, or the frosting melting and dripping off the cake.
Caramel icing is composed primarily of butter and sugar, both of which melt when exposed to heat. Additionally, a cake tends to crumble more easily while it is still warm because it has not set completely. Both these factors can result in a thin, messy layer of icing and a cake that lacks structural integrity.