Most of the women (except Andy) in The Devil Wears Prada. Nigel, too. Of course they do work for a fashion magazine and Andy does start "drinking the coolaid."
In A Knight's Tale Jocelyn likes wearing different clothes and dressing to match her knight because a flower is only as good as its petals.
Rachel in Animorphs is a Fashionista with dark Blood Knight tendencies, making her quite a well-rounded character indeed.
Alice in the Twilight saga. Loves dressing people up and throwing parties, especially if Bella's involved. She not only organized Bella's wedding, but packed her honeymoon suitcase, dressed her up in a cocktail dress and stilettos for her first post-vamp hunt, and filled up her closet at home mostly with high-end, fashionable clothing while Bella would rather wear jeans.
In Lonely Werewolf Girl this is the Hat of the Fire Demons. Standout character is of course Malveria, but Werewolf-fashion designer Thrix runs a close second. Showing up in the wrong dress could start a war you know.
Anyone who grew up reading The Baby-Sitters Club series will remember the obligatory description of Claudia's outfits as the highlight of the otherwise virtually copy-and-pasted introductory passage in each book. She was also the artsy one, and her dress sense obeyed no laws of fashion except her own, but we're assured that "on her, it totally worked" every time. Her best friend Stacey was from New York, which was treated like it was proof of fashion consciousness in itself.
Time Scout: Margo dresses well. Timeless class. And when she first encounters Connie Logan's shop, she practically orgasms.
The French, in British statesman Lord Chesterfield's Letters to His Son. "Fashion is more tyrannical at Paris than in any other place in the world; it governs even more absolutely than their king, which is saying a great deal."
The American version has Stacy London and Clinton Kelley.
Doctor Who: 200,000 years in the future, and the British What Not to Wear examples above are still at it as fashion-conscious robots!
C.J. on The West Wing, a bit, though it's also brought up as one of the ways she has trouble being taken seriously as a female White House press secretary (people either focus on how she looks rather than what she's saying, or assume she does that).
When earlier female American Idol judges started commenting on how a contestant looked, it was usually a sign that they couldn't find anything good to say about their performance. Nicki Minaj, on the other hand, is almost always giving commentary/suggestions on their style choices and still commenting on the performances themselves.
Monster High has Clawdeen Wolf. Unlike most fashionistas though, she is serious about going into the fashion industry in a business-minded way and in her 2011 doll with diary the first entry mentions her AP class in fashion.
Animal Crossing has roving fashion consultant Gracie, a giraffe. Gracie is a rare case being that he's male... in Japan.
One of the oldest examples is Yuko Asahina, in Tokimeki Memorial. This makes her one of the most special girls in the cast, as the secret to win her is to go to dates to the trendiest places or shows of the moment For MassiveRelationship Values.
Shiki from The World Ends with You has a talent for sewing and mentions bringing to life the designs of her friend Eri. She serves as a contrast to protagonist Neku who professes no knowledge of trends or fashion and barely notices damage to his own clothing. Although by damage, Shiki means "one of your buttons is slightly coming off."
Junko Enoshima from Danganronpa is a very famous kogal model known for her extravagant tastes and she has this as her Super High-School Level title. Except that she's not really Junko herself but her twin sister Mukuro Ikusaba disguising as her, as they made a Twin Switch before the start of the series. And the real Junko? She's the Big Bad, the mastermind behind the entire events of the series, which includes killing Mukuro and then using her corpse to fake a murder.
Self-proclaimed by Rhonda in Hey Arnold! and brought up in almost every episode she's in... despite her limited wardrobe.
Rarity, from My Little Pony: Friendship Is Magic, is an example of what happens when the management insist that a cartoon aimed at preteen young girls has to have one of these, but the showrunner is bound and determined to have the series pass the Bechdel Test. She dodges the stereotype by being a producer rather than a consumer: instead of being an obsessive shopper, she's a fashion designer who owns and runs her own boutique. As a result, most of her episodes focus heavily on her efforts to succeed as a fashionista and often deal with the business end of the job.
Stella from Winx Club is this, or strives to be this.
Cornelia Hale from W.I.T.C.H. tends to be this. She loves shopping for clothes . . . and shoes.
Marie Antoinette was supposedly a huge fan of fashion in her day, making her dressmaker, Rose Bertin, a celebrity of sorts.
WWE's Natalya. She was initially portrayed as a tomboy but that got dropped since she's not like that in real life at all. Just look at her shoe collection.
Peyton R. List, who plays the aforementioned Emma in Jessie, is this in real life. It was mentioned in an interview that Emma's original wardrobe prior to her being hired was cute but somewhat normal compared to Peyton herself. As a result the wardrobe people used Peyton's own style to help make Emma more of a true fashionista. They sometimes collaborate Peyton's clothes with the wardrobe choices for Emma to create even more outfits.