Fill cupcake cones halfway with prepared cake batter. Put a piece of foil over a muffin tin, cut a little hole for each cupcake and then pop the cones into the middle of each muffin section. The foil helps stabilize the cones while baking. Bake according to package directions for cupcakes (usually around 20 minutes).
Take a utility fork or skewer and poke the bottom of the cones to help them vent while cooling. Place on cooling rack to cool completely.
Frost and decorate with sprinkles. Push a mermaid tail into the middle of each cupcake cone.