The speaker of "The Highwayman" spends a lot of time talking about the way the main characters look. We get tons of detail about the highwayman's snappy dressing, and the speaker clearly wants us to know that Bess is good looking. On top of that, there's a pretty sharp contrast between the sexy main couple and the ugly, sloppy, almost sub-human appearance of Tim the ostler. We usually try not to judge people based on looks, but this poem almost demands that we do just that.
Bess's hair and eyes mirror the stormy darkness of the natural world in this poem. The appearances of the world and of our heroine are meant to create a visual rhyme that helps give the poem its structure.
The speaker of this poem plays deliberately on our prejudices. He relies on us to assume that beautiful people (like the highwayman) do beautiful things, while ugly people (like Tim) behave in ugly ways.