Prepare popcorn according to package directions. Remove all unpopped kernels.
Combine hot popped corn with dry drink mix in large bowl. Coarsely crush popcorn with hands. Mix 1/2 cup of the whipped topping into crushed popcorn. Press into bottom and up sides of 9-inch pie plate.
Gently spoon softened frozen yogurt into popcorn crust, spreading evenly on top. Top with the remaining 1-1/2 cups whipped topping. Freeze uncovered 3 hours or overnight.
Cut pie into 8 servings. Garnish with lemon slices, if desired.