Actually, wikipedia has a good definition of terms:
A bimbo isn't necessarily highly sexually attractive. Being a bimbo is a state of mind, and reflects a person who exaggerates the effort and value put into her physical attractiveness. She is often perceived as a shallowly focusing on her physical appearance and neglecting the development of other parts of her personality.
The word bimbo is sometimes interpreted as the backronym "Body Impressive, Brain Optional".
Good job anonymous wikipedia contributor!