"In private, Doreen called her Pollyanna Cowgirl"

This is the nickname given by Doreen to Betsy from Kansas to convey both her bubbly personality and rural background. 

Pollyanna is a popular children's novel by Eleanor H. Porter, first published in 1913, after which the term Pollyanna became a byword for any bright, cheery, optimistic, person.

A cowgirl is the female equivalent of a cowboy.