Remove the plastic insert to the ready-made graham cracker crust. Save the plastic insert to use as a lid for storing the pie while refrigerated.
In a medium bowl, whisk together the instant pistachio pudding and whole milk for 1 to 2 minutes or until it thickens. Set aside.
In a large mixing bowl on medium speed with a handheld mixer, beat the softened cream cheese and marshmallow fluff for 1 to 2 minutes or until fluffy and no lumps remain.
Add the prepared pistachio pudding to the beaten cream cheese, 4 ounces (half an 8-ounce container) of the thawed extra creamy whipped topping, and ¼ cup of the finely chopped pistachio nuts. Fold all the ingredients together until well combined.
Transfer the pistachio pie filling into the prepared graham cracker crust and smooth the top in an even layer.
Refrigerate the pie for 4 hours or up to overnight to allow the filling to fully set and chill.
Once chilled, you can garnish your pistachio pie by placing the remaining 4 ounces of extra creamy whipped topping into a piping bag or large plastic zip-top bag fitted with a piping tip and piping the whipped topping around the edges of the pie.
Sprinkle the remaining ¼ cup of finely chopped pistachio nuts over the top of the pie.