I usually prefer when the cosplay is faithful to the source but I also think the cosplayer should do whatever he wants. If he wants my approval sure, go for it... but if a black woman wants to do Sephiroth, power to her. Hell, it may even look good!
Not everything needs to appeal to everybody, and while I prefer things one way doesn't means I can't appreciate something different, and even if I don't like it also doesn't mean it should not exist. The cosplayer took his time and effort to make it happen, people should at least respect that.
With this all said, I want every fat guy doing magical girl cosplay to come with a in "shock" tag.