humorous if someone is not just a pretty face, they are not only attractive but also intelligent.
'How did you know that?' 'Well, I'm not just a pretty face, you know.'
be just a pretty face
To be physically attractive but lack any distinguishing achievements, intelligence, abilities, or other personal characteristics. Tiffany might be popular because of her looks now, but once we're out of college she's going to be just a pretty face.