In first scenes of the film, Audrey Hepburn as Sabrina has a ponytail— a girlish, 1950s sock hop-ready hairstyle. But when Sabrina meets the Baron, he tells her that in order to be a sophisticated Parisian woman,
BARON ST. FONTANEL: To begin with, you must stop looking like a horse.
This sequence was probably familiar to many viewers in 1954—because Hepburn did just about the same thing, with the same symbolic meaning, in her first big film, Roman Holiday, from the year before. In that movie too, her character grabs onto a new freedom and new stylishness by getting her hair cut very short. (Source)
Long hair is often seen as feminine, so it might seem odd that Hepburn keeps becoming womanlier by having her hair trimmed. But just because it violates notions of femininity, short hair in the films is seen as daring, unconventional—and therefore adult and sophisticated.
By being less feminine, Sabrina becomes more exciting and more adult. (That's a lot to think about the next time you get your hair cut.)