Revealing mistake: When Mombi's headless body awakens and sits up, in the first two shots it is easy to notice that her body is actually simply a puppet of some sort, as the motion is not very fluid and it looks like a mannequin. Once she stands up, she becomes an actress whose head has been removed with special effects.
Character mistake: When Princess Mombi is sleeping she is snoring, but she has no head.
Answer: She was in the human world so she was in disguise.