A female with an attractive body, but a "marginally" attractive face(hence the use of the word "margarine"). A margarine face is a female whose face barely exceeds the minimum requirements for an attractive face, as not be ugly. A margarine face is slightly more attractive than a butter face, as a butter face has no facially attractive features whatsoever.
I saw a girl with a great body the other day, but her face wasn't all that attractive. She was a total margarine face.