Can you use rolled buttercream icing on a dummy cake? ?

yes you can...

you want to make sure though that the surface of the dummy cake is smooth...if not, the rolled icing will show all the bumps, etc and will not look smooth...

if it is not a smooth surface or is bumpy, you need to put a smooth layer of royal icing over the dummy first...and actually, spackling like you get at the hardware store works too...

wait until it dries enough to be tacky but not completely dry, then apply the rolled buttercream...this will help it stick...

if you do not need to use something to smooth the styrofoam, just use a damp cloth and rub it over the styrofoam to dampen it a little then apply the rolled icing...

i think it's a great choice when you want the look of a large wedding cake without all the leftover cake or just to have your wedding cake to keep :)

You can do rolled fondant or buttercream... you can't roll out buttercream. Buttercream can be applied very smoothly to mimic fondant... and that's what I suggest. Fondant looks nice, but really tastes nasty, IMO.

Rolled Buttercream Icing