Preheat oven to 375 degrees F (190 degrees C). Spray a baking sheet with cooking spray.
Season chicken thighs on all sides with salt, pepper, and garlic salt.
Pour corn muffin mix into a shallow bowl. Roll chicken thighs in corn muffin mix until well coated. Transfer coated thighs to prepared baking sheet.
Bake chicken thighs in the preheated oven until no longer pink at the bone and the juices run clear, about 35 minutes. An instant-read thermometer inserted into the thickest part of the thigh, near the bone should read 165 degrees F (74 degrees C).