Every once in a while you might see a bulletin announcement or other form of communication taking folks to task about their clothing at Mass. Messages like this become more prevalent in summer, when warm weather fashions are sometimes immodest, and people might unthinkingly come to Mass dressed in distractingly revealing clothing.
Interestingly, immigrant populations usually do not need to be reminded to dress up for Mass. Sunday congregants attending Mass in foreign languages are often better-dressed than those speaking the native language of the place. Speculating, I'd imagine that one of the reasons for this has to do with work. It is not easy to learn a new language and culture, and to enter into the work force at a high level without specialized skills. Often, newly arrived folks work at jobs that require casual work clothes, or uniforms. Thus, Sunday Mass is one of the few public situations where a person can express freedom in clothing. Dressing up is fun, when you do it once a week.
For people with white-collar jobs, the situation is reverse. Every day, a suit and tie. Every day, stockings, heels, earrings, and a scarf. So on Sunday, dressing up would not be so clearly an expression of freedom; in fact, the reverse is true. A person expresses freedom by wearing relaxing clothes in public wherever that is possible. Khakis and short sleeves represent freedom.
In one way or another, I think, the time spent at Mass ought to be different from time spent at work or shopping or any other activity having to do with our earthly advancement. (Those whose workweek includes Sunday Mass know how difficult keeping the otherworldly focus can be.) The time-out-of-time of the Liturgy should represent the weekend in a special way, that time free from "unnecessary servile work." This doesn't need to mean dressing for the dance club or the beach; there are modest ways to dress for Mass.
I tend to think that the way to go, for women, is prettier than would be strictly appropriate for most work situations. More prints, more dresses, less structure, more flow--and much less black.