Most of the humor in Nickel and Dimed: On (Not) Getting by in America is based on attempting to survive with some sense of personal dignity and value intact. The only way to deal with some of the ridiculous and/or demeaning situations Ehrenreich faces is to laugh.
There is irony in some of the descriptions of her actions. As an exhausted waitress attempting to care for customers who are "hardworking locals" not coming to the Hearthside Restaurant for a fabulous meal, Ehrenreich quickly becomes protective of her customers, leading to humorous connections.
if you wonder why Americans are so obese, consider the fact that waitresses both express their humanity and earn their tips through the covert distribution of fats.
The irony of the restaurant business arises at several points in the chapter.
Picture a fat person's hell, and I don't mean a place with no food. Instead there is everything you might eat if eating had no bodily consequences - the chees fries, the chicken-fried steaks, the fudge-laden desserts - only here every bite must be paid for, one way or another, in human discomfort.
Most of the humor in the chapter, and the book, is a survival technique.
In Ehrenreich's "Serving in Florida" chapter, the author's humor is ironic, as the restaurant, far from being a clean haven, is disgustingly dirty. She writes, using an extended metaphor,
"The kitchen is a cavern, a stomach leading to the lower intestine that is the garbage and dishwashing area, from which issue bizarre smells combining the edible and the offal: creamy carrion, pizza barf, and that unique and enigmatic Jerry's scent--citrus fart."
The scatology in this passage makes it darkly funny, disgusting, and ironic. The restaurant, which is supposed to be clean, is very close to the feces and garbage it creates. She connects the food the restaurant serves, such as pizza and citrus, to gross bodily functions.
The humor in the chapter is also sarcastic in that the author expresses contempt for the people she meets in her job as a restaurant server. For example, the author writes, "Customers are, in fact, the major obstacle to the smooth transformation of information into food and food into money--they are, in short, the enemy." She makes fun of the "Visible Christians," people who come in after church services and leave her paltry tips, not a very Christian action. The customers only present her with headaches, and she uses sarcastic humor to express her contempt for them.