Social status in eighteenth-century England was fixed for the most part, meaning that if one were born in the gentry class or the aristocracy, they would most likely stay there. By the same token, a peasant would never rise to a higher class than the one into which he or she was born. Since Moll was born to a thief in Newgate Prison, the fact that she believes she could ever become a "gentlewoman" is laughable to ladies of the upper classes.
This kind of social stratification is often unfamiliar to contemporary American audiences, since our culture generally believes that a person can raise their social status by gaining weath, working hard, marrying a higher class, etc.
Because of the class status, for ladies,Moll would not be a gentlewomen.