Whilst she looks happy and contented, there is something not quite right about her expression. Perhaps the artist sensed a somewhat eccentric side to her character and did his best to communicate this to the viewer of this drawing. It is hard to imagine that she was a supporting character to another artwork because she would draw all of the focus.
It may have been that the artist spent a few days drawing portraits of local people for the pure enjoyment of doing so and that this lady stood as someone with character and interesting facial features. The likes of Durer and Bruegel were particularly interested in the lives of the poor, at a time when many artists were more interested in depicting the lives of the rich and well established members of society. Bruegel himself was famously nicknamed as Peasant Bruegel because of this.
These slightly perculiar expressions are often used by artists when depicting the insane, where a whole new set of facial portraits becomes available as a result of the unstable nature of their behaviour. Theodore Gericault and Edvard Munch are perhaps the most famous artists for broaching the topics around mental health within artistic portraits.