Maka cares, if the sudden straightening of her posture and the tilt of her chin are any indication. Maka has a sense of humor, she just doesn't like being the butt of the joke, and doesn't like it when her attempts at clarifying get laughed at.

Why is she questioning him? Because he's clearly just an idiot trying to be funny. She stops the questioning and just... flips him off.

(Lots of her arguments devolve into that now, but it's more effective than trying to communicate everything she wants to say).