In my opinion, what the author is showing here is that Miss Martha Meacham is giving up her hopes for romance. This is shown by her taking off her fancy clothes (the blue dotted silk waist, which means a blouse or shirt) and putting back on her plain clothes for working.
To me, this is mainly a story about a woman who lets her desire for romance push her into believing that this man needed her help. She built up this picture in her head of what he was and why he needed the stale bread. In doing so, she created a fantasy world where he was going to become her man.
At the end of the story, she has found out how wrong she was and she is going back to her previous life. This return to her normal state is symbolized by the change of clothing.