It's not that Megumin's magic is weak. It's not. Darkness has that much defense, magic defense, and HP.

She'd be a great crusader except her accuracy/hit percentage is atrocious. Her only saving grace is that she actually enjoys being hit as their party's tank.

The only reason Megumin seems weak is that the only spell she knows is Explosion, and she constantly boosts it when she levels so she can never keep up her MP with its cost.