You know, looking at this image, I think I may have subconsciously incorporated a little of Mrs. Ichinose from Maison Ikkoku into Mombi's character design.

For Mombi's traveling outfit, I considered giving her a huge bonnet like she did in Neill's illustrations, but I decided to go with a pointy hat as that's the most popular hat in Oz and made it white because she's a witch.

The bow makes her look more like Mother Goose than a wicked witch, but in her own mind, she's a good witch.