Mathilde is a dynamic character, but only to a certain extent. While she experiences the conflict of losing the necklace and her good looks, emotionally and mentally she remains shallow.
Essentially: A dynamic character changes, and a static character does not change.
Mathilde is a flat character emotionally. Although she goes through emotional pain and changes physically, a character needs to develop and change emotionally in order to be truly dynamic.
Consider the conversation she has with Mme. Loisel at the end of the story. If Mathilde had truly changed, she would no longer blame her problems on others. When Mme. Loisel comments that Mathilde has changed, look at her response.
“Yes, I have had hard days since I saw you, and many troubles,—and that because of you.”
Mathilde may have changed physically because of the ordeal with the necklace, but she did not learn from the experience. She did not grow as a person. She might have been changed by having to work, and losing her beauty, but she did not become a better person for it.