Preheat the oven to 375°F (190°C).
Spoon 1/2 cup of the jarred tomatillo salsa into a 13x9-inch casserole dish and spread it evenly to coat the bottom.
Place the 4 boneless, skinless chicken breasts in the casserole dish in a single layer.
Season the chicken evenly with 1 teaspoon garlic powder, 1/2 teaspoon kosher salt, 1/4 teaspoon ground cumin, and 1/4 teaspoon ground black pepper.
Pour the remaining tomatillo salsa (about 15.5 ounces) over the seasoned chicken, covering each piece.
Sprinkle 4 ounces shredded mozzarella evenly over the top of the salsa-covered chicken.
Bake uncovered for 30–35 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the cheese is melted.
Remove from the oven and let rest 5 minutes before serving.