Heat a large skillet over medium-high heat. Add sausage; cook and stir until browned and crumbly, 5 to 7 minutes. Drain and discard grease. Stir in celery; cook and stir until softened, about 10 minutes. Transfer to a slow cooker.

Stir zucchini, diced tomatoes, bell peppers, onion, salt, sugar, oregano, Italian seasoning, basil, and garlic powder into sausage mixture.

Cook on Low for 4 to 6 hours. Garnish each serving with 1 tablespoon Parmesan cheese.