Cook the macaroni in boiling salted water following package directions. Drain and rinse in a colander.
Heat the oven to 350 F (180 C). Butter a 2 1/2- to 3-quart baking dish.
Meanwhile, heat butter in a saucepan over medium heat. Add the flour and continue cooking for 2 minutes, stirring or whisking constantly. Gradually add the milk, whisking to blend thoroughly. Cook, stirring, until thickened and smooth.
Stir in the ground mustard, salt, and freshly ground black pepper. Add 2 cups of sharp cheddar cheese and cook, stirring, until melted.
Combine the drained macaroni with the cheese sauce, blending well.
Heat the pulled pork on the stovetop or in the microwave until hot (or as package directs if you use purchased).
Spoon the macaroni and cheese mixture into the baking dish. Top with the pulled pork. Sprinkle the pulled pork with the remaining cheddar cheese.
Bake for about 20 to 25 minutes, or until the casserole is hot and the cheese has melted.