The problem with fighting barefoot is twofold:
1) Ever stub your toes? Ever had someone in a shoe (or just in general) step on your bare foot? Hurts. That’s the problem with fighting barefoot. No armor. No protection. The average (non sandal/non heel) shoes constitute some basic protection for your foot both on the bottom and on the top, more so depending on the kind of shoes you wear. Someone stamping toes/instep/general foot is very common in combat situations, especially when it devolves into a standing grappling tussle. In this situation, you don’t want to be the one who ditched their shoes and ends up fighting the guy that’s still in theirs. In fights, you take every advantage that presents itself and feet are easily accessible. They’ll grind those toes. The benefit of the pain and worry they cause is that their victim will start looking down, start worrying about their feet more than they normally would. This takes their eyes off the other person, giving them more openings in their defenses via distraction and more chances to take them down.
The foot trap is also a common means of preventing movement with your opponent. It involves one person capturing the other person’s foot with theirs, applying pressure to keep them in place while they work them over. This will hurt a lot more under a sole than it might while still being protected by a layer of cloth, plastic, or (best case) leather.
It’s not even fighting dirty. It’s fighting smart. Men and women stomp each others feet. Men and women pull each others hair. When you’re fighting for your life you take all the advantages you can.
2) Ever gone running on asphalt barefoot? Running through a forest? Running on grass? There are plenty of natural hazards out there for a foot used to being protected by shoes. Shoe soles will get you over broken glass where bare feet won’t. It’s not just the person they’re fighting that they’ve got to worry about. It’s also their environment. They need to be more careful. Calluses earned on a mat will only get you so far. Fighting involves a lot of wear and tear on the body, and it induces a lot of friction on the feet. This is less of a problem indoor, where the floors are smoother (unless you’re on a rug because ouch), but it’s a problem outside. (This, of course, applies less to a person who wears no shoes at all in general has turned their feet into an approximation of hardened leather. Those feet aren’t going to be pretty though.)
3) Socks. You lose traction. Socks are designed help your feet inside shoes, but they provide almost no traction on their own and they’ll keep your feet from gaining the necessary friction when moving. They slip down, tangle up, and if you’re on a hardwood floor then, well, it’s going to suck. You’re better off barefoot, because at least your feet aren’t working against you.
It’s worth remembering that tearing up your feet is preferable to being dead.A person with bare feet fighting a person in shoes has exposed a
vulnerable part of their body to their enemy and to their environment.
Honestly, it’s not something anyone is going to want to be doing unless
they absolutely have to. If your character is forced into it, then her
priority should be on disengaging as fast as possible. If she knew she was going to be in a fight, then setting up a contingency where she has better shoes (and a better outfit) to switch to isn’t a bad idea.
The tight dress will hinder her movement, especially her legs when she has to brace. She’ll want to hike it up as high as it will go, all the way up to her waist and pin it if she has time (or can). If she doesn’t, the fabric will start to slide back down her legs because, well, physics.
If I’m in a tussle with a man and we’re both gripping each other by
the upper arms in an attempt to yank each other off our feet and I don’t
spread my legs wide enough or drop deep enough to keep my balance
giving him an opportunity to yank me off my feet, should I condemn him
if he takes the opening? Should he be expected to politely refuse to
take the advantage I’ve left for him while we’re grappling with each other
for our lives?
If he doesn’t take it and I win
the fight because of it, then it’s his fault. If I lose the fight because he saw an
opening, went for it, and succeeded then it’s mine. In a tight dress, this is going to be a real problem. Any form of dress which restricts the body’s movement is going to be one. When you fight, you’re going to slip, you’re going to slide, you’re going to knock into foreign objects, you’re going to get grabbed by the hair, grabbed by the head, and more than likely slammed into a few walls on the way down. Clothed or naked, you’re going to tear up your body. That’s what fighting is. More exposed skin just means cuts, more bruises, and more obvious tells to your moves.
The real problem I have with authors dressing characters is when they want to make a claim for realism but then choose to ignore the detriments of the choice. They either expect the dress sense not to affect the fight one way or the other or expect the opponent to not take the advantage when it’s offered to them on a silver platter. And it is a choice, it is a character choosing to do it this way instead of any other. They are choosing to go into battle with a handicap. Accept it, embrace it, and work up from there.