I told my girlfriend she drew her eyebrows too high...

I told my girlfriend she drew her eyebrows too high.
She seemed surprised.

The humor in this joke is a little bit hard to define. In western culture (and perhaps all human cultures) raising ones eyebrows is a facial expression that indicates surprise.

If someone told you that you had painted your eyebrows too high it wouldn't likely be a comment that would make you that surprised - it might make you angry, but not surprised. The reason that he commented that she seemed surprised, then, is that the fake eyebrows were too high, so he seemed like she was raising them. This is sort of silly, because if he knew that she had painted on her eyebrows too high, then why would he have assumed that she was surprised?

This joke is either really bad, or really funny, but I can't tell which.