Although she is of the lower classes herself, married to a gas station owner and living in the Valley of the Ashes above the gas station, when she is in New York City with Tom, Myrtle puts on the airs of a fine lady. As Nick describes it, after entering the apartment Tom has rented for her, Myrtle changes into "an elaborate afternoon dress of cream-colored chiffon." This leads to a change in personality as Myrtle acts the part of the grande dame: Her "vitality" converts to an "impressive hauteur." She becomes more and more "affected," Nick says. 

It is strange--ironic--that somebody of Myrtle's lower-class origins, who is merely the mistress of wealthy man and completely dependent on him for what finery she has, would speak so dismissively of the servant class: "These people! You have to keep after them all the time," she cries when Tom orders her (as if she is the servant) to get more ice and mineral water. 

It's not uncommon for people to denigrate the very class they have just risen (or fantasize about rising) out of, as if to distance themselves from the reality of their own lives. Myrtle is much like Gatsby in many ways--in her aspirations to rise above her origins, her "vitality", and sadly, her death due to getting mixed up with the Buchanans, but we see in Gatsby greater charisma and generosity: he may, like Myrtle, use affected speech, such as the "old sport" phrase that so irritates Tom, but Gatsby doesn't denigrate the servant class.

In referring to the employees of the apartment building where Myrtle and Tom Buchanan conduct their fantasies of being happily connected to each other instead of to their legally recognized spouses, Myrtle comments that "most of these fellas will cheat you every time. All they think of is money." Later, she expresses her disgust with the room delivery service.

"I told that boy about the ice," Myrtle raised her eyebrows in despair at the shiftlessness of the lower orders. "These people! You have to keep after them all the time."

The irony of the comment is that Myrtle Wilson spends most of her time thinking about money - specifically how much of it Tom has and how she would like to spend it. In her marriage to George Wilson, she has very little money or any of the luxuries it can make possible; she is one of the "lower orders" she disdains. When she is in the apartment, however, she elevates her status - at least in her own mind - to that of one with the financial resources to buy

A massage and a wave, and a collar for the dog, and one ofd those cute little ash-trays where you thouch a spring, and a wreath with a black silk bow for mother's grave that'll last all summer.