To call her Princess Diana wasn't correct, in fact. She was Diana Princess of Wales to Charles' Prince of Wales however. Just as is the case with Catherine, Princess William, or Meghan, Princess Harry. Both now have the title Duchess (of Cambridge and of Sussex respectively) which means they are not called Princess anything. The common tabliods/magazines continue to call Diana Princess Diana, wrongly.
Women outside the Royal bloodline, are not entitled to be called Princess, other than via the name of the Prince they marry.