I usually use a ratio of 2:5 butter to icing sugar and then stir in a bit of milk until it's a pipable, smooth consistency. 80g of butter to 200g of icing is usually enough for about 10 cupcakes if you pipe it.
Do you want to spread the icing? I find that piping gives better results presentation wise.
Not sure how you'd get it smooth using frosting but if I was frosting a full sized cake, I usually just apply the icing with a palette knife and move it in circles to give decorative swirls.
If you smear a bit of buttercream on a cupcake then use a large cookie cutter to cut out a round of pre-rolled fondant icing and use your hands to smooth it over the cupcake you might get the effect you're after