All I did to get the top of the cake to look like this was by placing the spatula on the top of the frosted cake. I tilted it slightly and lifted up to get an icing peak. I went another place on the cake and did it again. I continued until the entire cake top to get this look.
Of course, a smooth top is perfectly acceptable. If you let the cake sit iced for a couple of hours, it'll form a slight crust. Once it's got that crust, place a large piece of waxed or parchment paper loosely over the top of the cake. Gently, rub your hands over the top of the cake to smooth any ruff areas. You can do the same thing on the sides.
Basic Cake Making
How to Make Buttercream Icing
Frosting and Icing Recipes