I really liked Willow's wardrobe. In the early seasons it was child-like, girly, playful and, frankly, adorable. It made her dorkiness even more charming. Then, as she grew and progressed and really got the hang of the being a Wicca thing her clothes became more down-to-earth and Tara-like but still fit her, as she became more and more witch and less computer gal.
All I can say is, it fit her character and its development. Had she worn clothes like Buffy or Cordelia she would lack a lot of charm and come off as a mere copy.
Also, her dress in Once More With Feeling. I love it. It was so swishy and pretty and I want it! x)
After all, BtVS is not a show which makes its actors look their best but tries to make the characters seem more real and alive, unless many others. Which says a lot. A show about a vampire slayer and her gang of equally more than ordinary friends that is more believable than your average high school drama.