Mombi was not exactly a Witch, because the Good Witch who ruled that part of the Land of Oz had forbidden any other Witch to exist in her dominions. So Tip's guardian, however much she might aspire to working magic, realized it was unlawful to be more than a Sorceress, or at most a Wizardess.

The Marvelous Land of Oz, Chapter 1, "Tip Manufactures Pumpkinhead" (1904) - Later Oz novels