I would have to expand the previous answer a bit. I think that Myrtle is part of a larger social statement that Fitzgerald is making about superficiality. The entire array of characters is rooted in superficiality and the idea that surface is accepted as the real. Jordan Baker is one who continually fawns over the superficial, knowing that this is the only reality she wishes to pursue because of its immediate gratification. Tom is only drawn to superficiality. He wishes to present himself as a simplified intellectual with his inane theories of race and ethnicity, and his affair with Myrtle is based on the superficial momentary gratification. There is nothing substantial to Tom, and if there is, it is brutality and violence. The party scene to which Gatsby acquiesces in order to redefine himself is one based on superficiality. The parties, the catering of food, the opulence and splendor are all superficial. There is little substantial to it. There is little grounded in it. In all of this, the statement that is being made that the social context in which so many, including Fitzgerald himself, participated in the 1920s was one where superficiality and a sense of surface "gloss" reigned supreme. It was something that infected all of those that actively participated in it. In this, Fitzgerald's statement is that individuals who reveled in this held some level of responsibility for the cataclysmic plummet that happened at its end and the 1930s economic depression that followed.
The character who most represents "superficiality" is Myrtle Wilson, George Wilson's adulterous wife. For starters, she's disappointed in her husband for seemingly no reason other than that he had to borrow a suit to get married in. When Tom takes her and Nick to New York City, her hasty no-thought purchases make it clear how shallow she really is: a gossip magazine and a dog, who all but languishes once it arrives at the ritzy love nest, like the dog biscuit that slowly decomposes.
Myrtle serves to show how shallow and superficial the rich can be. Although not rich herself, she wants others to think that she is, a task made nearly impossible by being married to a working-class guy like George. She is attracted foremost by Tom's wealth.