Mix ground beef, saltine crackers, onion, green bell pepper, milk, egg, Worcestershire sauce, garlic, black pepper, and seasoned salt together in a bowl.
Stir ketchup and brown sugar together in a bowl. Spoon ketchup mixture into the bottom of each muffin cup of a 6-cup muffin tin.
Fill muffin cups with beef mixture, leaving 1/2-inch space on the top.
Bake in the preheated oven until no longer pink in the center, about 30 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C). Drain fat from muffin cups.
Top each 'cupcake' with mashed potatoes and Cheddar cheese.
Continue baking until cheese is melted, about 10 minutes.