She dresses down only for class, really. Around the dorm, or with keitaro alone, she dresses cute to sexy. Ditto for when shes around Seta.
It seems like she wants top flaunt it when around friends or loved ones, or when said people can benefit for her (such as her bathing suit choices in the series). I think there is a major conflict between parts of her personality though, the part that wants to put off major relationship issues and runs when things get tough, the part that wants to show off her killer figure and be sexy, and the part that seeks to punish perverts/etc. She's a flighty sadist exhibitionist, and that's actually a pretty bizarre combo.
Motoko was much easier to figure out. She distrusted men, and thus lashed out at their perversions, until she herself found a man worth loving. In the manga, once she falls for Keitaro, she becomes way, WAY nicer to him, and no longer punishes him in the same way. Granted, it's not till she loses him entirely that she finally breaks down and confesses, unable to deal with the heartache.
Also note how Naru loved to flaunt Keitaro once she won him. Making out in the hotsprings in front of everyone? really? She's indeed quite possessive. The only one probably more possessive of him in canon was Kanako.