Heat the sugar and 1/4 cup water in a medium saucepan over medium heat and cook until the sugar has dissolved, about 3 minutes. Add the chopped mangoes and let steep for 2 minutes. Add the ground cardamom and fennel and blend with an immersion blender until completely smooth. Measure 2 cups of the puree and set aside. Transfer the remaining puree to a large bowl and allow to cool completely.
In a medium bowl, whip the egg whites to stiff peaks. In a separate bowl, whip the cream to soft peaks. Gently fold first the egg whites into the cooled mango puree and then the whipped cream. Refrigerate until cold.
Grind the cardamom and fennel seeds in a mortar and pestle and stir into the reserved mango puree. Fill the bottom half of each of 3 martini glasses or parfait cups with the puree and then fill the rest of the way with the chilled mango mousse. Garnish with a sprinkle of pistachios.