Yes you are right about the sour expression and how she is smiling very faintly.

The woman that he painted was in an unhappy marriage. She was 16 when she got married to a 32 yr old widower. So you can assume the troubles of that much of an age gap. Her smile is faint and her eyes are blank.

And the smile is of course not perfect as compared to Mona Lisa because it was quite the beginning of his career.