He'd a French cocked-hat on his forehead, a bunch of lace at his chin, (line 7)
The highwayman is meant to cut a pretty dashing figure. His clothes are the first thing that let us know he's a stud. Think of him like a sharp-dressed gangster, a gunman in an expensive suit. Whatever you think about the way he makes a living, you can't help but think he's kind of cool. In this poem, it's pretty safe to judge a book by its cover.
But the landlord's black-eyed daughter,
Bess is as beautiful as the highwayman is well-dressed and cool. They make a great pair. Since the poem doesn't have time to show us how compatible they are, appearances make a great shortcut. We don't learn a lot about Bess, but it's important that she's so dark (dark hair, dark eyes, etc.). That rhymes nicely with all the other dark moments in the "The Highwayman" (like in line 1) and it generally fits with the moody intensity of the poem.
Where Tim the ostler listened; his face was white and peaked; His eyes were hollows of madness, his hair like moldy hay, (line 20-21)
This is a really awesome two-line portrait. We've heard all this stuff about how sleek and beautiful Bess and the highwayman are. Now we get this – a ruffled, crazy-looking guy, almost melting into the hay of the stables. We love the word "peaked" in line 20. It's hard to say exactly what it means to have a peaked face, but it gives us a sharp image of Tim's pinched, deformed, ghost-like head. This description is also sort of sad and brutal, and it tells us all we need to know about Tim's love for Bess. It ain't gonna happen. We know by their appearances that Noyes thinks she's way out of his league.