In a medium bowl, combine crushed graham crackers, granulated sugar, and melted butter. Stir until the mixture resembles wet sand and holds together when pressed. Spoon the mixture into individual serving cups, pressing firmly into the bottom to form a sturdy crust.

In a large mixing bowl, use an electric mixer to beat the softened cream cheese until smooth and creamy, about 2 minutes. Gradually incorporate powdered sugar, mixing thoroughly after each addition. Add the fresh key lime juice and optional lime zest, continuing to mix until well blended.

Carefully fold the whipped topping into the cream cheese mixture using a spatula until there are no streaks remaining.

Spoon the key lime filling over the graham cracker crusts in each cup, smoothing the tops with a spatula. Cover with plastic wrap.

Refrigerate the cups for at least 2 hours to set properly.

Once chilled, remove the cups from the refrigerator. Top each cup with a dollop of whipped topping and a sprinkle of lime zest.