The reader can appreciate the transformation in Della's appearance because O. Henry devoted so many words to describing the abundance and beauty of her hair before Madame Sofronie hacked it all off. For example:

So now Della's beautiful hair fell about her, rippling and shining like a cascade of brown waters. It reached below her knee and made itself almost a garment for her.

When Della gets home with Jim's present and without her hair, she immediately starts trying to do something with the few inches of hair she still has left. She ends up with tight little curls all over her head, held in place with hairpins. In addition to looking like "a truant schoolboy," she comes up with this analogy:

“If Jim doesn't kill me,” she said to herself, “before he takes a second look at me, he'll say I look like a Coney Island chorus girl. 

Evidently there were a few women who were already starting to wear their hair short at that time. O. Henry's story was published in 1905. Chorus girls might have actually started a fad. They had to work hard for long hours and didn't have the time or the inclination to be caring for the long hair that domestic women like Della could cultivate. The increasing number of women who had to work in the outside world would be tempted to get rid of all that superfluous hair.

F. Scott Fitzgerald wrote a story titled "Bernice Bobs Her Hair" which memorialized this significant change in women's fashions. It was published in 1920. There were many other radical changes in women's fashions, women's attitudes, and women's behavior during the Roaring Twenties. 

The Nineteenth Amendment, which prohibited state or federal sex-based restrictions on voting, was ratified by sufficient states in 1920.   -Wikipedia