Ok, I'll bite (no pun intended), why is it an insult?
Those outside of the fashion industry call anyone who is "into fashion" or "wearing crazy clothing" as being a fashionista.
Those inside of the fashion industry view the term "fashionista" as being more equated to a label whore or someone who tries (well, some might actually live it) to portray a lifestyle of luxury/glamour/taste. Think the FIT student who runs around with a monogram Gucci tote, Dior aviators and runs around to the main designer boutiques to buy a tshirt or sneakers here and there. Their myspace or AIM name will probably have the word "couture" in it.