The landlord's red-lipped daughter, (line 23)
This little extra description of Bess is just a reminder of how beautiful she is. Coming right after the lines about Tim, this contrasts her sexiness with his frightening oddness. The narrator never comes out and says that Tim ratted the highwayman out to the soldiers, but we think it's the only explanation. Why else would Tim be there? The difference in the two men's appearance adds the first bit of tension to the poem. You can see how this love triangle would be explosive, how Tim's bitterness would cause him to do something crazy.
As the black cascade of perfume came tumbling over his breast; (line 35)
More material about how attractive Bess is. This is all done for a good reason. If we feel the connection (both physical and emotional) between these two, it makes the tragic ending all the more difficult and intense. The speaker uses appearances, and the idea of these beautiful young lovers, to make us feel more invested in the plot of the poem.