It's quite a leap to say that Maou isn't strong in combat. We really don't know what she is capable of doing.

The Hero looks pretty scrawny, but he's the only human we've seen using magic at all, and he is considered a beast of a fighter, reportedly several magnitudes stronger than Lady Knight.

Head maid doesn't look suited for combat either, and not only is she a General (who presumably must keep the more...obviously strong monsters in line under her command), she uses summoned ghosts/spirits to attack.

I suspect Maou's magical abilities have been carefully hidden, because utilizing them doesn't serve her goal, and it was to her benefit to look powerless when convincing the Hero of her plan.