Preheat oven to 350°F and grease a 9x13-inch baking dish.
In a large bowl, combine the chicken, diced zucchini, cream of chicken soup, sour cream, salt, and pepper, then spread mixture into the prepared baking dish.
Sprinkle the shredded cheddar cheese evenly over the chicken mixture.
In a separate bowl, toss the dry stuffing mix with the melted butter until well combined.
Sprinkle the buttered stuffing mixture over the top of the casserole.
Bake uncovered for 45 minutes until the stuffing is golden brown and the casserole is bubbling around the edges.