Heat a large skillet over medium-high heat. Cook and stir beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain and discard grease. Stir in sauce, mushrooms, and olives. Cook and stir until heated through, about 2 minutes more. Transfer mixture to a 2-quart rectangular baking dish. Sprinkle mozzarella cheese on top of the casserole.