Heat coals or gas grill for direct heat. Spray half of one side of four 18x12-inch sheets of heavy-duty aluminum foil with cooking spray.
Brush fish lightly with oil. Sprinkle with salt and cumin. Place fish on sprayed side of foil sheets. Mix salsa, corn and tomato; spoon onto fish. Fold foil over fish so edges meet. Seal edges, making tight 1/2-inch fold; fold again. Allow space on sides for circulation and expansion.
Cover and grill packets 4 to 5 inches from medium heat 10 to 15 minutes or until fish flakes easily with fork. Place packets on plates; unfold foil. Sprinkle with avocado.