I think it depends on what kind of frosting you're making. A meringue-type frosting is soft enough you can dip and twirl the cupcakes. In others, you have to apply with a spatula. But what specific problem are you encountering? Like, is the cake breaking as you frost it? You can't get it even or to look nice? A heavy buttercream made from butter and powdered sugar might need a little cream in it to spread easily. But you can always put it in a decorating syringe or bag and squeeze it through a large hole with the zigzagy edges (the kind that makes ridged snakes of frosting come out). You just apply it in a thick spiral from the outside towards the inside and make the end peak upward by pulling up the syringe at the end and stopping squeezing it.
Also, mixing by hand was the only way once upon a time, so it does work, though it requires a lot of elbow grease! I used to do everything by hand before i got my mixer.