Lou, all by itself, is become fashionable for girls, which usually makes a name LESS fashionable for boys. But Lou, like Bill or Jim, is rarely used on its own for boys anyway. (You wouldn't name a boy Frederick Lou the way that girls are named Mary Lou or, in the case of Keri Russell's new baby, Willa Lou.) Long form Louis is getting cool again and, with the Lewis spelling, is the Number 2 name in Scotland.
Lou is usually a short form of Louise, Louisa, or Lucy in English-speaking countries, when Lou is used for girls at all. But in France and Germany, it's a fashionable choice all on its own, sure to gain even more widespread style credibility since Heidi Klum and Seal chose it for their daughter.