Personality is one thing, appearance is another.
Getting your hair regularly cut at that length requires effort, to the extent of going out of your way to a barber shop or salon and asking for them to give you a boy's haircut like that.
Many guys around here on TC probably have longer hair then her, since it takes effort to get a haircut, and none to let it grow out.
Hell, she could wear a wig while the hair grows out, she doesn't want people on the street to recognise her anyway.
As I said, wearing a skirt only as a special occasion and making a big deal out if it is not really trying to be more feminine, people change their normal manor of dress for special occasions/events all the time, such as weddings, funerals, attending church, dates or even the recent Halloween.
That doesn't mean they're going to make a regular habit out of it.
Here's a simple guide on how she can change her wardrobe.
step 1, go to a store and buy tons of women's clothing, but nothing that looks remotely male.
step 2, toss out all old clothing.
step 3, there is no step 3 because it's that freaking simple.
And she's a famous idol now, there's no reason why she can't afford to do this.
It's not like men in anime don't do this sort of thing all the time you know.