Line a 4 1/2-by-8 1/2-inch loaf pan with parchment, leaving a 4-inch overhang on all sides. Prepare a large bowl of ice water. In a medium saucepan, combine coconut milk and condensed milk and bring to a boil over high. Reduce heat and cook at a rapid simmer, stirring frequently, until thickened, 10 minutes. Remove from heat and stir in lime zest and salt. Place saucepan in bowl of ice water and stir until mixture is cool, 4 minutes.
In a large bowl, using an electric mixer, beat cream on high until stiff peaks form, 2 minutes. Gently fold whipped cream into coconut milk mixture. Pour into loaf pan; evenly sprinkle coconut over top. Freeze until firm, 6 hours (or, covered, up to 3 days). Let sit 20 minutes at room temperature. Invert onto a serving platter; peel away parchment. Layer mango on top and serve.
In place of mango, you can use thinly sliced fresh pineapple as the tropical topping.