Should cakes be frozen and dirty iced before laying fondont? Also, how to avoid and lumps and bumps from show through fondont?
Cakes don't need to be frozen, but I find it easier to bake and freeze the cakes ahead of time, then level and fill while they are still frozen, and then crumbcoat and cover in fondant (they are still firm by this point). In order to not have any lumps showing through the fondant, you must smooth the icing completely before applying the fondant. A crusting buttercream or ganache works best for this. HTH!
The cake does need a crumb coat, but I just refrigerate it for several hours or over night. The best way to avoid bumps is to have a smooth finish on the crumb coat. Also something I have done from time to time is instead of on thick fondant layer I cover the cake in two thin layers. The first layer helps to smooth a bit then top it with the second layer and it should be a perfect finish.
I will usually put my cakes (with a thin smooth layer of buttercream) in the freezer for 10-15 minutes. Its not really necessary but I find that it helps with the fondant application if the cake is really cold.
NEVER put fondant on a frozen cake. Trust me. But yes, you should crumb coat and apply extra icing if you want. I always put a regular coat of icing under my fondant since I found out that I could.