What becomes a legend most? For JANE FONDA, it turns out to be the rote romantic farce "Monster-in-Law" — a cannily selected vehicle that's more than funny enough to RELAUNCH her as a MAJOR MOVIE STAR after a 15-year layoff. Fonda is a hoot and a half as Viola, a Barbara Walters-esque TV diva who schemes to separate her son from his fiancée, Charlie. (Jennifer Lopez) Monster-in-Law" is first and foremost a TRIMUPHIANT COMEBACK for Fonda, whose acting chops complement a presence that puts most of today's stars to shame.