I’m going to avoid talking about the cultural context for the skirts, which there is and just focus on the practicality.
The answer to any question involving combat is “it depends”, and when we talk about an article of clothing that is dependent on that specific article of clothing. It also depends on the kind of combat you plan to have your character engaging in. Street brawls are very different from armored melee. If your character is a female soldier, she’ll be dressing according to whatever regulations her military has (that could involve a skirt for dress uniforms, but battle and dress are different).
There is no “one-size fits all” approach as the field of battle matters, the kind of opponent matters, the skill level of all parties involved matters, context matters. What your intentions are matter.
They all factor into the decision making process. What you need to do when looking at articles of clothing and trying do decide if it’s a yes or no is learn to think from the internal perspective of someone who would actually be engaging in physical conflict. If you’re thinking of someone heading into a dangerous situation where they couldn’t outwardly look like they were expecting trouble then the question is: if you expected to be caught and forced to fight, what kind of clothing would you prefer to be caught in?
It starts with you and we work our way out from there as you learn more about the conditional nature of combat. When it comes to Hollywood, the irony is that most of the clothing male action heroes wear will work for basic street combat whereas the clothing for women won’t. Would you want to be hunting monsters through the sewers in six inch heels? Probably not.
For what the girl gangs are doing, it works. In fact, it works better than a miniskirt or any other tight clothing common for women in the US or the leather bondage outfits you often see women fighting in on television. You’ll still see women in the real world wear those. Not because it works, mind you, but because they’re afraid they won’t be perceived as feminine, sexy, or attractive. They overcompensate in the wrong direction, the same way Hollywood and media do, and for the exact same reasons.
Sometimes, people make choices that have nothing to do with what’s appropriate or what works. Sometimes, they’re trying to balance between societal expectations, cultural mores, gender constraints, and what they’re trying to accomplish. Sometimes they’re trying to be outside the box and inside the box at the same time. And, sometimes, they can get away with it. What they’re doing and who they’re fighting means they’ve a greater margin for error, versus someone faced with an enemy where they need every advantage they can get.
What you want, especially with street fighting, is freedom of movement.
This is why you often see tank tops or very loose fitting shirts on military personnel. If you’ve got a shirt that fits tightly around the shoulders, that’ll impede your movement, restrict the rotation of the shoulder. If you’re pants are too tight or limit flexibility, then that slows you down and will limit how high you can kick, how well your leg moves, etc.
You want durable clothing.
Clothing that protects you in a fall or when you’re rolling around on the ground. If you can’t see it absorbing impact or protecting you from scrapes when you hit the earth, then it isn’t a good pick.
You want clothes that breathe.
Combat is a high energy exercise, it’s frenetic, it’s fast, and it takes a lot of exertion. If you’ve ever brought the wrong kind of clothing when you’re going jogging or watched makeup melt off girls in P.E. class then you know what I’m talking about. Clothes that cause you to overheat, that don’t allow the heat to escape your body, that you can’t run or sprint in, will actively do you harm in a fight. By participating in exercise with a high energy output, you are already heating up your body. (This is part of why we sweat, we’re cooling our body down.) The hotter you get, the faster you burn through your water. The hotter you get, the faster you reach a point of critical exhaustion which will get you killed.
However, “what works” for combat is heavily dependent on the kind of combat your character plans to (or potentially might) engage in. The rules change based on what you’re doing, what you need, what the chances of success are, who the enemy is, the terrain you’re fighting on.
There’s also the other side, beyond practicality, which is you know, cultural expectations and considerations. How your character feels about gender norms, whether they care about being perceived as feminine or masculine, whether they care about expectations, whether they’re vain, or willing to get themselves killed over fashion.
There’s also the part in fiction where how someone is dressed becomes an indicator for how serious the situation is/threat level is. That’s a visual tell you see used often in film and television.
Remember, skill and experience don’t free you from the same constraints that affect other characters. They just mean your character can make more intelligent choices based on what they know. They can get away with more, but it will still catch up to them in the end.
So, be Helen Mirrim in R.E.D. and take out armed gunmen with a reinforced clutch and the element of surprise.
Try thinking about the situation from the perspective of the character involved rather than overall generic rules. Practicality changes on a situational basis, and there are plenty of people who will go Rule of Cool in real life. This is especially true of gangs, where efficacy loses out to intimidation.
People are people. All the factors going into a decision may not be the ones you expect or are looking for.