Preheat oven to 350ºF. Line muffin tins with 18 mufifn liners and set aside.
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.
Scoop batter into muffin liners, filling about 3/4 full. Bake for 18-20 minutes, until a toothpick comes out clean. Let cool on a cooling rack.
While cupcakes are cooling, bump up oven temperature to 400ºF. Spread coconut on a baking sheet and bake for about 5 minutes, or until toasted, stirring every so often so it doesn't burn. Pull out of oven and set aside.
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.
Frost cooled cupcakes and sprinkle with toasted coconut. Now, devour! :)