Dutch Disney comics feature a Madame Mim story roughly once every month. None of these mention Arthur or Merlin, and they all take place in the Present Day. She's friends with Magica De Spell. Mim has a bad case of Villain Decay in these stories, to the point that some other writers have pretty much thrown her origin as a villain out of the window and made her a good guy.
It happens in Italian Disney comics as well, where she keeps her Italian name of 'Maga Magò' ((Female) 'Magician Magicò').
Best Known for the Fanservice: if you look up Madam Mim on a photo website of any kind, you'll notice that half her pictures will be her in her beautiful lady form instead of her usual old self.
Draco in Leather Pants: As mentioned before, Madame Mim's popularity in comic books often renders her a good guy. In fact, when she is friends to Magica de Spell, she is invariably the Token Good Teammate. Most of the stories she's in deal with people seeking her help, with the people in question being various fairytale characters or obnoxious villagers who blame Mim for everything that goes wrong. Also, she's still often mischievous and evil-ish, such as when dealing with a government official looking to tear down her forest to make way for a road.
Let's just say that the movie is not in fact about a nameless female squirrel's doomed romantic pursuit of a shape-shifted Arthur.
Arguably, Madam Mim herself. While she's officially the main antagonist, she's also a Plot-Irrelevant Villain who appears during the third act without any previous mention or foreshadowing and has roughly ten minutes of screentime. However, her entertaining and creative Wizard Duel with Merlin made her one of the most well-known and well-remembered part of the movie.
Fan-Preferred Couple: It goes without saying that the girl squirrel would've been far better to Arthur than Guinevere was.
Fanon: The Hazel story◊ from 4chan's /co/ (a bit suggestive, so probably NSFW). Many /co/mrades accept it as legit canon just because they want that squirrel to have a happy ending.
Germans Love David Hasselhoff: The Dutch love Madame Mim. She's one of the most often seen Disney characters in the Netherlands. She's pretty popular in Sweden as well, where people who know her invariably learned about her from the comics.
Robot Chicken would make their own take on giving the girl squirrel a happy ending, where Wart convinces Merlin to turn her into a human despite being warned she'd retain her squirrel mind. Also, he gets her pregnant.
Smurfette Breakout: A rare phenomenon of a film having two examples. The first is the female squirrel who is left heartbroken when Arthur turns back into a human. She's one of the most remembered things about the film, and many a Fix Fic are written in which she gets a happy ending. The second is Madame Mim, who only appears towards the end, and yet became a mainstay in the comics. She even Took a Level in Kindness to become slightly less evil as a result of her popularity.