Add pudding mix and cream cheese to a large bowl and beat until smooth.
Add milk a little at a time until completely mixed the cream cheese.
Using a rubber spatula, fold the Cool Whip into the cream cheese mixture until evenly combined.
Place a layer of graham crackers in the bottom of an 8-inch x 8-inch baking dish. Break the graham crackers as needed to fill in any gaps without overlapping.
Cover the graham crackers with 1/3 of the pudding mixture and spread it out edge-to-edge in the dish. Top the pudding with 1/3 of the strawberries and blueberries.
Repeat layering the graham crackers, pudding, and berries two more times until you reach the top of the dish.
Cover the cake with plastic wrap and chill for at least 4-6 hours, up to overnight.
When ready to serve, cut the cake into 6 slices and plate. Garnish with a sprig of mint and serve immediately.