Preheat an air fryer to 400 degrees F (200 degrees C). Spray the air fryer basket with cooking spray.
Combine flour, salt, and pepper in a bowl. Pour ranch dressing into a second bowl. Combine bread crumbs, Parmesan cheese, and garlic powder in a third bowl.
Working with as many zucchini slices as will fit in the air fryer basket in a single layer, dredge zucchini in flour mixture, then shake off excess. Dip into ranch dressing, then shake off excess. Finally, press into bread crumb mixture to coat both sides. Place into the prepared air fryer basket in a single layer.
Air-fry until browned, 5 to 6 minutes. Repeat with remaining batches.