Preheat oven to 350°F (175°C).
Spread berries evenly in a 9x13-inch baking dish.
Sprinkle sugar, cornstarch, and lemon juice over berries; stir gently to coat.
In a bowl, stir melted butter into dry cake mix until mixture resembles wet sand.
Sprinkle topping evenly over berries, breaking up large clumps with your fingers.
Bake 45-50 minutes until edges bubble and top is golden brown.
If top browns too quickly after 30 minutes, cover loosely with foil.
Let cobbler rest at room temperature for at least 15 minutes before serving.