Thaw the frozen whipped topping in the refrigerator before beginning. Keeping it cold but soft makes it easy to fold into the lemon mixture without flattening its airy texture.

Whisk the sweetened condensed milk, fresh lemon juice, and optional lemon zest in a mixing bowl until smooth and slightly thickened. Strain the lemon juice first if needed to remove seeds and excess pulp.

Gently fold the thawed whipped topping into the lemon mixture until evenly combined. Use a broad spatula and stop mixing as soon as no large white streaks remain so the filling stays light.

Spoon the filling into the prepared 9-inch graham-cracker crust. Smooth the top with the back of a spoon or an offset spatula for an attractive finish.

Cover the pie and refrigerate it for at least 4 hours, or overnight when possible. Slice it cold and garnish each piece with lemon, extra whipped topping, or a small sprinkling of zest.