Why is the book called Stardust and not just Tristran Goes After A Fallen Star or something along those lines? Well, for one thing, it would sound way less magical. And since things get pretty magical up in this book, it's appropriate that the title give us some sense of this. Plus, Yvaine is a star and a major player when it comes to the plot, so the title's a bit of a shout-out to Tristran's leading lady.
But let's focus on the dust part for a moment. Dust is the stuff that settles around us, the tiny particles that get knocked about and stirred up by us as we go through our lives. And Yvaine, who we can see as the star in the title, sets a heckofa lot into motion when she falls from the sky. What she sets in motions, then—we're talking major quests for a number of characters—can be seen as the dust that gets stirred up by her dramatic plummet away from the moon. And in this light, the entire plot is sort of stardust. The title is a subtle—but totally pointed—reference to this.