Why is Robin never seen making funny faces? I have been seeing One Piece for a long time. I came to notice that during any funny scene, Nico Robin is never seen to make any kind of funny or weird face while all the other Straw Hat crew members do.

Did Oda at some time mention this specifically?
Nico Robin had a lonely childhood. Her mother was out on her expedition to uncover the secrets of the Void Century. Her only friends during her childhood days were the scholars of Ohara, and Saul for sometime.

During her time with Saul she was shown to smile and laugh in a funny manner.



But soon, at the age of 8, after the extermination of Ohara along with her friends and mother, she was on the run with a bounty of 79 million beri.




She was always targeted by anyone who found out about her true identity. This is a very harsh childhood for a kid. Enough to make a kid a "serious" type and steal their humor. 

Also, during the Enis Lobby arc, she mentions her fear:



It is evident from this that she was always over thinking things, and putting a lot of burden on herself. She kept her sadness and worries to herself, and in such a situation it is very rare for a person to make funny expressions, rather they have a cold look.