A “character foil” is a character who functions to create contrast with another character. For example, action movies often use a cowardly comic character to act as a foil for the hero, while comedy movies often have a “straight man” to be a foil for the goofy foolish character. Juxtaposing such strikingly contrasting character emphasizes those differences, making the action hero more heroic, or the comedian more funny.
The juxtaposed items don’t have to be in a perfect 1:1 balance. In the above example, the overall impression is one of dream-like perfection, marred only by the jarring appearance of the “wad of hair.” Still you could say that the “wad of hair” is juxtaposed against the “sugared air” (the rhyme also helps link the two) or the “porcelain” shower–the clean white place where it lies.