Preheat oven to 375°. Spread the coconut on a baking sheet; toast about 5–7 minutes or until golden. Set aside.
Combine powdered sugar, 1 tablespoon water, lime juice, 1/4 teaspoon lime zest, and a dash of salt in a small bowl. Let stand 10 minutes. Pour glaze over cake, sprinkle with the toasted coconut, and let stand 10 minutes or until glaze is set. Stir together mango, granulated sugar, and remaining 1/4 teaspoon lime zest in a large bowl.
Use a serrated knife to cut the cake into wedges. Arrange on dessert plates with the mango slices on the side.