Preheat the oven to 350 degrees F. Coat a 9-inch-square, 2 1/2-inch-deep cake pan with cooking spray. Prepare the cake mix as directed; pour into the pan and bake about 35 minutes. Let cool, then remove from the pan and freeze 30 minutes.
Trim the domed top of the cake with a serrated knife. Mark one-third and two-thirds of the way down the sides of the cake with toothpicks. Slice the cake into 3 thin, even layers using the toothpicks as a guide.
Make the lettuce: Put the coconut in a zip-top bag and add 1/2 teaspoon water and 10 drops each of green and yellow food coloring; shake the bag to mix. Add more food coloring as needed to make a lettuce color.
Make the tomatoes: Trim the straight edges of the gummy fruit slices with a paring knife to remove the sugary coating.
Make the bacon: Line up 10 cherry, 4 vanilla and 6 original Tootsie Rolls on a microwave-safe plate lined with wax paper, as shown.
Microwave the Tootsie Rolls until just soft but not melted, about 7 seconds. Press them together, then roll and stretch into a long thin strip.
Cut the strip into 4-inch pieces, then mold each piece into a wavy bacon shape with your fingers. Repeat to make another batch of bacon.
Spread the frosting on the bottom and middle cake layers; top with the coconut. (Leave the third layer unfrosted.) One at a time, cut all 3 cake layers diagonally into quarters.
Top the bottom cake quarters with some candy tomatoes and bacon. Add the middle cake quarters, then more candy. Top with the unfrosted cake quarters. Skewer with long toothpicks.