Preheat the oven to 190°C / 375°F. Lightly grease a 23 x 33 cm / 9 x 13-inch baking dish with cooking spray or olive oil.
In a large bowl, combine the shredded chicken, softened cream cheese, buffalo sauce, ranch seasoning, cheddar cheese, green onions, black pepper, and garlic powder, if using. Stir until the filling is evenly mixed.
Cut the tops off the orange bell peppers. Remove the seeds and white membranes from inside the peppers.
Use a small sharp paring knife to cut triangle eyes, a triangle nose, and a simple mouth into the front of each pepper to create jack-o’-lantern faces.
Stand the peppers upright in the prepared baking dish. Carefully trim a very thin slice from the bottom of any pepper that will not stand upright.
Spoon the buffalo chicken mixture evenly into the peppers. Do not overfill.
Sprinkle the mozzarella cheese over the top of each stuffed pepper.
Cover the baking dish loosely with foil. Bake for 15 minutes.
Remove the foil and bake for 5 to 7 more minutes, until the cheese is melted and lightly golden and the filling is hot.
Let the peppers rest for 5 minutes. Serve warm with ranch dressing, blue cheese dressing, celery, carrots, and extra buffalo sauce.