Line 12 cupcake cups with foil liners. Stir graham cracker crumbs, pecans and melted butter together in a bowl to combine. Spoon 1 Tbsp. of the mixture into each liner, pressing it down to make the crust.
With an electric mixer on low speed, beat the cream cheese in a large bowl until creamy, about 30 seconds. Add the condensed milk and key lime juice, and blend on medium speed until just combined, 20 to 30 seconds. Add 1 cup of the whipped topping and blend on low speed for 15 seconds longer.
Spoon mixture evenly into liners. Cover pan with plastic and freeze until firm, at least 3 hours. Remove from freezer 15 minutes before serving; peel off liners and dollop 1 Tbsp. remaining whipped topping on each cake. Garnish with lime slices, if you like.