As opposed to a butter face--a girl with a hot body but an ugly face--a margarine face is a girl with a hot body, but an average face. A margarine face is better than a butter face because margarine is not as bad for as you butter.
Dude, I was walking behind these two girls on the sidewalk and they had the most amazing asses I've ever seen. But then I saw their faces. The girl on the left was ugly as sin. TOTAL butter face! However, the girl on the right wasn't as ugly, so she was a margarine face.