Heat a large skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Mix celery into cooked sausage; cook and stir until celery is softened, about 10 minutes.
Combine sausage mixture, zucchini, tomatoes, bell peppers, onion, salt, sugar, oregano, Italian seasoning, basil, and garlic powder in a slow cooker.
Cook on Low for 4 to 6 hours. Garnish each serving with 1 tablespoon Parmesan cheese.