Symbolism, Imagery, Allegory
Yes, that's right. Hamlet gets flowers, Beowulf gets swords, and Holes gets body odor. As far as symbols and imagery go, it may not be the grandest or most poetic, but it's definitely one of the most fun. From the stinky foot fungus that Clyde Livingston and Trout Walker share to the powerful smell that comes off of Stanley and Zero as they drive away with Ms. Morengo, there are a lot of pretty gross body smells going on in this book.
Mostly this image is played for laughs – how can we not smile when we hear about a character whose "two feet smelled like a couple of dead fish" (23.10)? But these odors also remind us that there's often a big difference between what we think of something based on first impressions (e.g., smells), and what it actually turns out to be once we know more about it.