I am sitting here this afternoon, writing to you in a sweaty shirt and yoga pants, having just taken a four mile walk around Eagle Lake in Acadia National Park. The day was glorious, and I feel great. But if I were a character in a book, just exactly would it say about me that this is what I am wearing? I guess you could surmise that I enjoy exercising. But that would be incorrect. I hate exercising! However, I do enjoy a good walk on a lovely day. If, as a writer, you wanted to express that, you might have your character wear a pair of sturdy shoes and a wide-brimmed hat - these items would far better describe someone who enjoys a good walk in the sunshine while not necessarily being an exercise freak.
Clothes are an important element in helping your reader gain insight to your character. Think about it. Can you imagine Pigpen in a pair of designer jeans? Likewise, would Christopher Robin look right in a tuxedo? So while you as a real person may change it up(no pun intended) when writing, your characters are not allowed that luxury. Everything they wear are clues to your reader as to what to expect: who your character is and what makes them tick.
So think carefully about how you want your character to come across to your reader. If you want your character to be graceful and charming, a pair of capris with holes in them would not be the best choice of clothing. If you want your character to appear old and crotchety, don't let them listen to music on an IPod. It just wouldn't make sense. In the world of writing, clothes (and accessories) do, indeed, make the man!