Is Joy/Hulga static or dynamic?
This is a great question. The answer turns out to be complicated.
Joy/Hulga does not undergo a change with clear demonstration of the ways she has changed. However, she does have a transformative experience at the end of the story. Readers can reasonably presume that in the implied future for Hulga, she will be changed, though she does not show the effects of her experience within the context of the story.
The change in Hulga's character can be described as a movement from false confidence and pride to humility. She realizes how wrong some of her ideas really are.
The purported wisdom expressed by Hulga and by her mother in this story becomes ironic in the end and is proven to be folly. Mrs. Hopewell believes that "good country people" are the salvation of society, to some degree, and she identifies Manley Pointer as a good country person. He turns out to be a thief and a violent, calculating man.
Hulga's views on Manley are also turned on themselves.
Although she fantasizes that she can give him a “deeper understanding of life” by taking “all his shame away and turn[ing] it into something useful” by seducing him, it is he who seduces and shames her.
In learning the difficult lesson that she was quite wrong about Manley Pointer, Hulga learns an important lesson about herself. Her intellectual confidence does not extend to a sure knowledge of other people. She also clearly does not understand herself as well as she believed she did before having her leg stolen by a traveling salesman.
We might conclude that Hulga is a dynamic character insofar as she does learn a lesson that promises to change her, though the effect of this change will manifest itself in the implied future and not within the context of the narrative of "Good Country People".