Prepare popcorn according to package directions. Remove all unpopped kernels.
Combine hot popcorn with dry beverage mix. Coarsely crush popcorn with hands. Mix 1/2 cup 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 remaining 1 1/2 cups whipped topping. Freeze uncovered for 3 hours or overnight. Garnish with lemon slices, if desired.