Wicked is all about strong female characters: Elphaba, Glinda, Nessa, Sarima, Melena, Dorothy, Nanny, Nor, Morrible, Princess Nastoya... This legacy actually comes from L. Frank Baum himself, as Maguire explains:
"It was Baum who set up the powerful princesses of Glinda, the witches, and Ozma as the real wielders of power in Oz, and the Wizard was just a sham. Baum was an early and ardent feminist, as anyone who has read his biography knows. I think he'd have been delighted that Elphaba and Glinda (both in the musical Wicked and in my novels) are figures of power to admire, to emulate – and yes, as in any powerful figure, to question." (source)
Women are definitely figures of power and of influence in Wicked. And relationships between women also play a major role: mothers and daughters, sisters, friends, rivals, etc.
But the men aren't entirely absent either. Though we get some girl-power moments (often, interestingly enough, from the speechifying Morrible), there is also a lot of female oppression and an outright battle of the sexes in the book. Oz is a lot like an old-fashioned version of Western Europe or America in terms of gender relations. Women don't have equal rights, but men don't have it all that easy either (which we especially see with Turtle Heart). It's a prejudiced and oppressive world for everybody. But there's a particularly odd distinction between the sexism and lack of equality that women face here and the very real power and influence they exercise.