Whipped cream on cakes is delicious...but, won't stand up in heat. Here is a recipe for Stabilized Whip cream from Wilton.com
Stabilized Whipped Cream Icing
1/2 pint (1 cup) heavy whipping cream
2 tablespoons confectioners' sugar
2 tablespoons Wilton Piping Gel
1/2 teaspoon Wilton Clear Vanilla
Combine whipping cream and sugar in mixing bowl. Whip to soft peak stage. Add piping gel and vanilla, then continue to whip stiff peaks. Do not overbeat.
YIELD: 1 1/2 to 2 cups.
As an alternative, you can use Frozen Non-Dairy Whipped Topping or Packaged Topping Mix. Thaw frozen whipped topping in refrigerator before coloring or using for decorating. Use packaged topping mix immediately after preparing. Do not allow either to stay at room temperature, as it becomes too soft for decorating. Store decorated cake in refrigerator until ready to serve.