Not just a pretty face
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
This phrase is casual and mostly used in informal conversation or friendly contexts. 
When she solved the math problem, I realized she's not just a pretty face.
Pro Tip 2/3Complimentary Tone
Use it to compliment someone in a playful or light-hearted way, often as a surprise. 
Wow, you speak five languages? Definitely not just a pretty face!
Pro Tip 3/3Underestimated
Suggests the person has abilities or qualities beyond what others might expect at first glance. 
He's the best chess player here, and not just a pretty face.