In general, "madame" is used to show respect, presumably in lieu of qualifications that would allow the usage of the other respectful honourific, "Professor" (though it has been used to refer to known professors also); for example, this applies to Female Ministry of Magic employees like Dolores Umbridge and female members of the non-teaching staff at Hogwarts School of Witchcraft and Wizardry like Madames Pince and Pomfrey.