My perspective is that Rikku influenced Yuna to dress that way. That's just me though.

It also shows how much a person can change in 2 years. Although I don't think the change was THAT drastic. People do go through a change when they lose someone they love. Trying to rid their sub-conscience of memories of what they used to be. That's what I went through and from the looks of it, that's what Yuna went through, until she saw a sphere of him. After which she did what anyone would do and search for some answers. Possibly closure.