In a large bowl, whisk together the lemon instant pudding mix and whole milk for about 2 minutes until well combined. Once blended, gently fold in one container of thawed whipped topping until smooth and creamy.
Cover the bottom of a 9 x 13 baking dish with a single layer of graham crackers, breaking them as needed to ensure complete coverage. Spread half of the pudding mixture evenly over this layer of graham crackers.
Repeat the process by adding another layer of graham crackers on top of the pudding mixture, followed by the remaining pudding mixture spread out evenly across this second layer.
Top everything off with a final layer of graham crackers, then spread the remaining whipped topping over this last layer, smoothing it out nicely.
Cover the dish tightly and refrigerate overnight or for at least 4 hours to allow all the flavors to meld together and firm up properly.
Before serving, add fresh lemon slices and/or lemon zest on top for an extra burst of flavor and visual appeal, if desired.