You’re not supposed to like her. And yet, isn’t she just mahvelous?
She’s a rival of Merlin, and a minor antagonist of the film Sword in the Stone. Most of the trouble she causes is mischief, though. (She would eat you, you see, but it’s nothing personal.) The film doesn’t really answer who is really more powerful, Mim or Merlin, since Merlin has to resort to outsmarting her, but we only ever see her use one type of magic. (If she’d been able to do more and cheat, I’m quite sure she would have.)
In fact, she’s fairly amicable with Wart when he first dives into her chimney, and eagerly shows off to him.
It’s her contrary delight in all things gruesome and grim, and her distaste for wholesome, cheerful things like Cheerios that means she simply has to cause mayhem in Merlin’s plans for Wart.
Before she threatens to set England’s progress back another 200 years, though, she proves just how thin beauty goes, by transforming into a lovely young maiden, and a warty old pig. If I could get a perfect hourglass figure just by pulling on my hair, of course I would. Except, Mim knows best of all that it won’t change who you are inside. Notice that she doesn’t prefer to hang out in that form; she’s actually more proud of her hideous guises.
So, follow Mim’s advice and always be yourself. Unless you can be a purple dragon. Then be a purple dragon.