Preheat oven to 350 F. In a greased 13x9-in. baking dish, layer blueberries, sugar and marshmallows. Prepare cake mix batter according to package directions, adding lemon zest before mixing batter; pour over marshmallows.
Bake 35-40 minutes or until a toothpick inserted in centre comes out clean. Cool 10 minutes before inverting onto a serving plate. Serve warm with whipped topping.