Stir the graham cracker crumbs, melted butter, and granulated sugar in a bowl until the crumbs hold together when pressed.
Divide the mixture among four small tartlet pans or a muffin tin and press firmly into the bottoms and sides. Place in the refrigerator to firm while you prepare the filling.
Beat the cream cheese, pumpkin puree, brown sugar, vanilla extract, cinnamon, and nutmeg in a large bowl until smooth.
In a separate bowl, whip the heavy cream until stiff peaks form.
Fold the whipped cream into the pumpkin mixture until no streaks remain.
Spoon the mousse into the chilled crusts and smooth the tops.
Refrigerate the tartlets for at least two hours until set.
Top each tartlet with a dollop of whipped cream and a light dusting of cinnamon before serving.