The Heart is a Lonely Hunter
Much like her older sister Hazel, Etta is vain and materialistic. But here's the rub: Etta isn't that good looking, and she spends her time fretting over her looks:
She primped all the day long. And that was the bad part. Etta wasn't naturally pretty like Hazel. (1.3.49)
Mick's spoiled, superficial sister is also obsessed with Hollywood: she wants to be rich and famous and, of course, have rich and famous friends (don't we all?):
But Etta was like she was full of worms. All she thought about was movie stars and getting into the movies. [...] All she thought about was going to Hollywood when she could scrape up the bus fare and getting a job as a secretary and being buddies with Jeanette MacDonald and getting into the movies herself. (1.3.49)
The 1930s and 1940s are known as the "golden age" of Hollywood for a reason – in the pre-TV era, movies were the place to be. And this is especially true during the Depression, when movies served as a very welcome means of escapism.
Etta is a portrayal of a young woman in a modern, capitalistic society that emphasizes consumer culture and the entertainment industry (sound familiar?). But while Etta's dreams are materialistic, the fact that she fixates on bigger and better things links her to her younger sister, Mick, who also dreams of a more beautiful world and has her head in the clouds. These sisters seem to have different manifestations of the same condition: a desperate desire to escape the reality of poverty.
Women and Sickness
Etta is also distinguished by the fact that she's dangerously ill. Etta's ovarian cyst is just another in a line of reproduction-related illnesses in The Heart Is a Lonely Hunter: Mick has her own pregnancy scare, many of Doctor Copeland's patients shouldn't be having babies (due to economic or health concerns), and Alice's death represents a C-section gone wrong.
This focus on women's health acts as a comment on society and the role of women in that society. Women in this era were expected to have babies and be happy housewives, but Etta (and Alice, for that matter) falls apart physically and fails to live up to that standard.