Charlene has an 80's hairstyle -- if you lived in the era, you remember the tall bangs. It's part of the "theme" of the event. Princess Grace herself often attended the ball in costume and was criticized for her dress (and in once case had hair so tall she couldn't fit in the car). It's part of the part they play and simply a fun part of the theme.
She only attended once, back in 1969 when they had a hairdress contest and everyone had to wear one. The Rose Ball isn't a costume ball (masquerade) despite it's changing theme, sure some people like to wear something like an accesory related to the theme.
I think instead of it being the Rose Ball, to celebrate spring and the social season in Monaco, it's looking more and more like a Black and White Ball, regardless if they change the theme.
Everyone wearing black is kind of getting merged with the decor, lots of black tonight in the Sporting Club. I think Beatrtice Borromeo is the most beautiful of the night.