Where does this character come from? Searching for Madam Mim or Madame Mim seems to turn up nothing but Disney results. Searching wikipedia for Madam Mim does nothing but redirect to the cast of The Sword in the Stone.
Wickedpedia makes a claim similar to yours:
> Madam Mim appears in the original version of the novel, but not in the revised version featured in The Once and Future King
This appears to say that "the novel" refers to some other novel than The Once and Future King, but of course there is no primary Arthurian source in the form of a novel. I can't really understand it.
It's difficult to extend Wickedpedia the benefit of the doubt, because it goes on to say this:
> Kahl animated her [Madam Mim's] initial interaction with Arthur while Thomas oversaw her famous Wizards' Duel with Merlin. He is a witch Madam Mim who is more which than oversaw. He is also a tiger similar to Shere Khan from The Jungle Book, though he is not in a forest.