Why do you think Tom’s mistress is introduced exactly after the description of the valley of ashes?

By introducing Myrtle directly after the valley of ashes, Fitzgerald gives the reader a deeper insight into her character. Specifically, he reveals to us that Myrtle is using Tom to live her own American Dream. We see this clearly in the way she behaves when Tom and Nick go the apartment in the city. Although she is from a very poor and deprived area, she acts as though she is much richer and more affluent. She changes her clothes, for example, from a plain spotted dress into an "elaborate" gown made of "chiffon."
With the change of clothes also comes a change of personality. Nick notes, for instance, that:

The intense vitality that had been so remarkable in the garage was converted into impressive hauteur. Her laughter, her gestures, her assertions became more violently affected moment by moment.

In other words, Myrtle acts like someone who is from New York, not from the valley of ashes. She, therefore, creates the persona of the person that she dreams of being. Notice that even when Tom punches her, Myrtle does not break off their affair. She would never dream of doing such a thing because it is through Tom that she is able to escape the dreary world of the garage in the valley of ashes.