In a large bowl with a mixer, beat cheese and sugar until well blended. Add whipped topping, in dollops, beating to combine. Gently whisk in mango until blended.
Pour mango mixture into a 10- by 15-inch rimmed pan. Freeze until firm, at least 3 hours or overnight. When frozen, serve or wrap airtight and freeze up to 1 week.
To serve, cut into diamonds or rectangles and lift out with a wide spatula. Garnish with mango slices and pistachios.