Madame Thénardier is Monsieur Thénardier's partner in crime, and she acts every bit as terribly as he does, although we don't get to spend as much time with her as we might like. The main thing we learn about her is that she reads a lot a lot of romances: "Madame Thénardier had just sufficient intelligence to read books of this kind, and she devoured them, soaking in them what little mind she possessed" (18.104.22.168).
Brain snack: these aren't bodice-ripping romances, exactly—they're "classical" romances, so no explicit sex—but the implication here is the same: she's getting all kinds of ideas about the way her relationship and the world should be, and it's seriously messing with her head. As the old saying goes, a little learning can sometimes be a dangerous thing. Rather than improving Mme. Thénardier or giving her a pleasant escape from daily life, her reading makes her bitter, entitled, and mean.
The second main thing we learn about her is that she completely gives in to her husband, despite her strong personality. We read at one point that:
A new visitor to the tavern, seeing Mme Thénardier for the first time, invariably concluded that she was the real master of the house. It was a mistake. (22.214.171.124)
Thénardier is the real brains of the operation, and his wife is just a convenient partner in crime when he needs someone to, say, pretend that she's pregnant and sick. Madame eventually dies in women's prison while her husband escapes from men's prison. He doesn't really care about her death one way or the other, and at the end of the day, it's safe to say that Madame's life is just a sad bundle of bitterness and cruelty toward others without any redeeming spark of goodness.