Preheat oven to 350 degrees F (175 degrees C). Line a 9x13-inch baking pan with parchment paper.
Mix pumpkin, evaporated milk, sugar, eggs, and cinnamon together in a bowl; spread in the prepared baking pan. Pour cake mix on top. Pat chopped nuts into surface. Spoon melted butter evenly over nuts.
Bake in the preheated oven until golden brown, 50 minutes to 1 hour. Allow cake to cool completely, about 1 hour. Turn over onto a baking sheet; remove parchment paper.
Mix cream cheese, confectioners' sugar, and whipped topping together in a bowl. Spread onto cake.