Place potato nuggets in the bottom of a glass baking dish. Bake in the preheated oven until cooked through and crisp, about 20 minutes.
Cook ground beef in a large skillet over medium heat until no longer pink, 10 to 12 minutes. Drain off fat and discard. Stir in taco seasoning and water. Reduce heat to low and simmer until sauce is reduced and thick, about 10 minutes.
Spoon cooked beef mixture over the potato nuggets. Pour nacho cheese dip evenly over the beef. Top with canned tomatoes, olives, 1/2 cup shredded Cheddar cheese, and taco sauce. Bake in the oven until cheese is melted and the casserole is warmed through, about 10 minutes. Remove from oven and top with 1/2 cup shredded Cheddar cheese. Serve atop shredded lettuce and garnished with sour cream.