As far as style goes, those brightly colored, boldly patterned jammies and boxer shorts manufactured by San Francisco's Joe Boxer Corp. are the sleeper hit of the year. Leading the slumber party at the moment is fashion-conscious Kid of Kid 'N Play, who sports dynamic billiard ball-print sleepwear in House Party 2's rockin' ''jam.'' The movie's costume designer, Ruth Carter, says the pajamas have become a fashion statement offscreen as well as on. ''I used about a dozen pair in the film, in addition to Kid's,'' she says, ''and just about everyone took theirs home.''
In 1990, Joe Boxer made $13 million on the expanding pajama/boxer market. Other celebrities recently caught with their Boxers on: the perennially overexposed Madonna, who struck a pose in black-and-white polka-dotted undershorts in Truth or Dare, and Flavor Flav of Public Enemy, who wore golf ball-print pajamas to last year's MTV Awards. Roseanne Arnold, meanwhile, wears the ''Ponderosa'' print, featuring cowboys and bucking broncos, from New York's Shady Character Ltd., as does Murphy Brown's Candice Bergen, who will model her new home-appliances pattern on an upcoming episode.