I’m pretty bad at frosting a cake. Really bad.
But I have a trick to make your cake look a lot more professional. Use a hair dryer to smooth out the clumpy icing. Gently blow it onto the cake (use the high heat setting but the low-blow setting!) and when the icing starts to get shiny, turn it off. Then, use a spatula to go over the cake, gently flattening the icing. Works like a charm!