I run a fashion website and am a guy. I'm not a fashionista, but something else, right? What would I be?
The word looks like it's based on Italian, which has in turn borrowed the term, to the Italian word is now also "fashionista"!
The "-ista" ending, eg "chitarrista" (guitarist), "pianista" "acquarellista" (watercolour painter), etc, doesn't change regardless of whether the person it describes is male or female. So. while in Italian a male teacher is a "professore" and female is a "professoressa", a male fashionista is still a fashionista, just as Elton John is a "pianista" or Miles Davis a "jazzista". So, in short, you are a "fashionista".
Best of luck for your website!