The title (like Goddess or Domina or many, many others) is an honorific that is used to show respect. It is something the Domme herself chooses and each Domme has their own rules about what are the acceptable ways to address that particular Domme (I know of many who hate being called Mistress and will severely reprimand or punish you for using it).
As Mistress Ayn stated it has nothing to do with marital status or even anything specific beyond what that Domme decides for herself.