Shit like this is why people generally get pissed off at anime.
Every single expression, intention, thought and feeling of an anime character, is explicitly and bluntly clear to the audience. A girl is in love? BLUSH. Someone is evil and obviously lying? Well, that maid's face. Yet, the characters themselves cannot notice these things, for whatever reason. The guy cannot comprehend the obviously tomato-colored girl likes him, and the kid cannot understand that the obviously evil person is an obviously evil person. The character here, is at least a child, but on the same token -- why make it so explicitly obvious to the audience, that the maid is full of shit, by illustrating her that way?
Because it's easy, I guess.