The vampires that we meet in Twilight tend to have an amazing fashion sense. Bella emphasizes how the Cullens all look like models due not only to their stellar physiques, but also in the way they dress. On the other hand, Bella constantly reminds us that she's a normal girl and judging by her own description of her wardrobe, her clothes seem average too. So there's a fairly simple division: humans = normal/dull dressers; vampires = stylistic superstars.
But take a look at the Epilogue, when Alice helps Bella get ready for the prom. Suddenly Bella is looking quite stylish (vampire-ish?) herself. Does this symbolize that she's fitting in with the Cullens more and more, or that she's destined to soon become a vampire herself? We'll have to read the sequels to find out...