She had a film career at a time when Hollywood was obsessed with the female breast... the larger the better.
When the film offers dried up Jayne took to the nightclub circuit.
Diana Dors career followed the same pattern.
It seems that Jayne became a bit too fond of the bottle and she was living a fast life with her new lover Brody she did the clubs in England too.
Her biog says she had a good upbringing and was very intelligent and talented on the piano.
Maybe the dumb blonde bit was just an act.
Maybe in Hollywood she was mixing with the wrong crowd (Anton Le Vey *etc)