Arrange 21 cupcakes in a tight triangle on a platter or cake board, using 6 cupcakes at the bottom. Add 2 cupcakes for the trunk. Break the graham cracker in half, spread chocolate frosting on the back and use to fill in the gaps in the trunk as shown.
Spread the remaining chocolate frosting on the bottom 2 cupcakes, using an offset spatula to add texture. Tint the vanilla frosting green; transfer to a piping bag with an open-star tip. Starting from the bottom of the triangle and working your way up, pipe the green frosting all over the cupcakes in small downward strokes to cover completely.
Add candies for ornaments. Flatten the gumdrop and trim into a star shape with a small cookie cutter. Place on top of the tree.
To make chocolate frosting for the tree stump, remove 1/2 cup vanilla frosting and beat with 3 ounces melted bittersweet chocolate.