Firstly make sure that the cupcakes are cooled completely before you store them. Secondly any frozen food when brought to room temperature will have some condensation due to the temperature shock. Simply take a kitchen paper towel and dab it on top to remove the water droplets or run a fan near the cupcake. However letting the cakes to stand at room temperature for a while so that the condensation has time to evaporate is the easiest trick.