Tres leches cupcakes ought to be frosted for both taste and appearance. Those little holes you poked into the cupcakes, which allow the three milk mixture to seep in, aren't the prettiest and frosting covers them up. For these tres leches cupcakes I use a very simple buttercream frosting that I pipe on. However, you can dollop it on with a spoon instead to save time. Or, you can just make whipped cream and spread that on with a knife life I do in my Tres Leches Cake with Whipped Cream. In a pinch, whipped topping like Cool Whip will also work just fine.