Blend the mango flesh to a puree in a food processor. Stir in the lime juice.
Heat the sugar and water in a saucepan over a medium heat for 2 to 3 minutes, or until the sugar has dissolved and the mixture resembles syrup. Bring the mixture to a boil, and continue to boil for 4 to 5 minutes, or until the mixture has reached the soft ball stage. The mixture has reached the soft ball stage when a sugar thermometer reads 235 to 240 degrees F when inserted into the mixture.
Gradually add the hot sugar to the whisked egg whites in a thin stream, whisking continuously, until all the sugar has been incorporated and the mixture is thick and glossy. You may choose to use gelatine for a firm set. If you are using it, place the gelatine leaves into a small pan and cover with cold water. Set aside for 4 to 5 minutes to soften, then drain well, squeezing any excess water out of the leaves, and return to the pan.
Add 2 tablespoons of water to the gelatine leaves and heat over a medium heat, until the leaves have dissolved. Set aside to cool slightly, then add the mango puree, and stir until well combined.
Fold 1/3 of the whipped cream into the mango mixture, or until well combined. Add the remaining whipped cream and the egg white mixture, and gently fold the into the mango mixture until well combined.
Spoon the mango mousse into a large serving dish or into individual serving glasses, and chill in the refrigerator for 2 hours.
To serve, sprinkle over the chopped pistachios.