1.Preheat oven to 350ºF. Line muffin tins with 12 muffin liners and set aside. 2.In a medium-sized mixing bowl, pour the cake mix. Pour the diet soda in slowly to the cake mix. Stir together with a whisk until all lumps are gone. 3.Scoop batter into muffin liners, filling about ¾ full. Bake for 18-20 minutes, until a toothpick comes out clean. Let cool on a cooling rack. 4.In a medium-sized mixing bowl, combine cream cheese, butter, powdered sugar, lemon zest, vanilla and lemon juice. Blend together with a mixer until all combined and smooth. 5.Frost cooled cupcakes and sprinkle with lemon zest. Enjoy!