Symbolism, Imagery, Allegory
Stachenfreude
Pink-haired author Laini Taylor is from Portland, and let's just say we can tell. This book's Portland hipster cred is totally cemented by all of the seemingly random references to mustaches. Karou meets Kazimir at a place called Mustache Bar... the only place where this tattoo would be cool.
Then, there's Izîl, who believes that "men of wisdom" (18.31) often have fine mustaches. His examples: Mark Twain and Nietzsche. Other than Izîl however, none of the characters in the book actually have mustaches.
Can we infer that none of them are actually "of wisdom", then? Well, maybe all the mustache talk is just Laini Taylor's attempt to land a cameo role on Portlandia. Or maybe male characters in the book want their mustaches to symbolize their stateliness—but, like in real life, you can't always get what you want…