Why a 57 mm gun fits the Constellation-class mission

Is the Constellation class undergunned with a 57 mm main gun?

No; its ASW and group-defense role is backed by other US ships carrying heavier guns, while the 57 mm supports local defense.

The Constellation class is intended chiefly for anti-submarine warfare and task-group defense rather than independent heavy surface action. The US Navy already fields numerous Burkes, Zumwalts, and future large combatants for missions needing a five-inch gun. The frigate's 57 mm weapon also continues the armament scale of the LCS it replaces. Its rate of fire can contribute useful defensive fire and put metal into the air without distorting the escort role by duplicating capabilities already present elsewhere in the group.