Everyone starts off with base stats. Kazuma had awesome luck/intelligence and not much else. The opposite is true for Aqua.

Wiz had way more MP than Megumin. Megumin is a poor choice for explosion magic - something that drains her MP in one go. She may not be a weak mage, but she chooses a shit way to go about it.

So yes, Megumin is a weaker mage than Darkness is a crusader because of how she applies herself.