He’s really good about displacing gender stereotypes.
He makes it clear that Holly is pretty; he then proceeds to shape her whole character as this autonomous, clever, badass motherfucker who will throw away the rule book and do what needs to be done to get shit done, and never brings up the fact that she is a ‘she’ or a 'pretty she’ except when shit hits the fan and she’s being a bamf and then usually it’s Artemis that reminds the audience “by the way, this woman is kicking ass and she’s DOING IT REGARDLESS OF HER LOOKS”.
He also writes Holly as a character who is clearly unashamed of her femininity. She grows out bangs (a purely aesthetic thing) and she’s affectionate. She has emotions and she sits in coffee shops. I’d be willing to bet that on her off days she sometimes wears skirts and she does say that unlike the boys, she uses moisturizer to keep her ears looking good.
Colfer essentially tells his audience that you can be a girl, you can be a pretty girl, you can be a girl who likes feminine things, and you can still take care of business.