Combine ground beef, breadcrumbs, and milk in a large mixing bowl. Add ketchup, onion, bell pepper, eggs, Worcestershire sauce, garlic powder, salt, and black pepper. Mix until well combined.
Transfer the mixture into a loaf pan and shape it into a loaf. Spread remaining ketchup on top.
Preheat oven to 350°F. Bake the meatloaf for 60 minutes or until the internal temperature reaches 160°F. Let it rest for 10 minutes before slicing.
Boil cubed potatoes in a large pot until tender, about 15-20 minutes. Drain and return to pot.
Add sour cream, butter, and milk to the potatoes. Mash until smooth and fold in cheddar cheese. Season with salt and pepper.
Serve sliced meatloaf with a generous scoop of cheesy mashed potatoes.