I was thinking about the role of the author's emotional state and its effect on writing, and was going to make a comparison to martial arts, but then I got distracted by my socks.
What's so special about my socks, you ask? Well, they are black and neon yellow. I have an entire collection of socks that are black with accents of neon color. I absolutely love each and every single one of those socks. Why? Because they're fun, they're cheerful, and they make me happy when I wear them.
Last night, I was having a discussion with a friend about suits. We were commenting on the fact that if a suit is well-made, and fits well, it makes the wearer look fantastic. We then drifted into a conversation on how when we wear suits, we feel kick-ass and confident, because we know we look good.
I think, to a certain extent, everyone wears a certain kind of clothing to get into a particular mental space. Wearing comfy lounge pants makes me feel relaxed and cozy, wearing a dress makes me feel pretty, and so on and so forth. As much as our clothes can make us feel safe or sexy or competent, there's also an inverse relationship. If I'm tired, I will probably put minimal effort into my outfit, whereas if I'm excited/nervous, I will put more effort into what I'm wearing.
While perhaps putting this much thought into what your characters are wearing would be overkill, if they have certain things they do with their clothes, it can reflect their personality, social standing, emotional state, etc...
For example, I always intended Riley to be a little bit ambiguous gender wise. She has a very neutral name, wears what would normally be designated as "male" clothing, and has short hair. To me, all of this is a wall presented to other people, to make her unapproachable. She wants to be tough, strong, independent, and so she wears what she perceives to be more practical clothing. (In writing this, I'm wondering if I should have more people do double-takes when they meet her.)
Kael I wanted to be a lot softer of a character, personality wise. He's very bookish, and was never much for violence or practicing combative styles. He wears more relaxed outfits, although it's all well made and decorated and what not because he's a prince. Mostly, this manifests in his habit of not wearing shoes when walking around outside. If he's travelling, then yes, he will wear shoes, but if he's at home and wants to walk around in the garden, off go his shoes. I also imagine him spending a lot of time indoors without shoes, because it's just more comfortable for him. It's a rejection of social expectations of his birthright, and also speaks to his desire to be comfortable, rather than to follow what is socially acceptable.
Minor characters also express personality in their choice of clothing, or in their lack of choice of clothing. While I don't advocate judging people on appearances, you can learn a lot about them and where they're at based on how they present themselves. It certainly is worth thinking about.