Forgive me if my comments are not entirely germane to the topic, as I'm struggling somewhat with parceling out what that topic is when statements along the lines of "You can argue in favor of simple character design's merits all you like, but the sheer amount of Shadow fan art indicates . . . something" are presented.
In any case, I should like to state that a character's design may be as alluring as possible, but if the character themselves is unlikeable, there isn't much for it.
Take, for instance, the following:
Very few people will argue with you that Lightning is a
bad character
design. Most all of us were rather hooked before XIII came out. Now, as someone who liked her in XIII, I still must say that design not withstanding, there are plenty of people willing to tell you that Lightning isn't great, and that she's the reason they don't go near the FNC, FF itself, or Square, or whatever.
The point is, if the character on its own is so repulsive it trumps the good design, then no matter how good the design, its effect is utterly reversed because of the character, and that previously good design now stands as nothing but shorthand for the dislike people feel toward the character.
Now, as to the argument that fan art is somehow indicative of quality of one design type over another, allow me to extend the Final Fantasy example one step further out. There is plenty of fan art dedicated to the Warrior of Light, in his very simple red armor, floating out there. Perhaps not as much as Lightning, for the obvious reason that his Amano art is more well known of late, but the point remains, you can find swathes of fan art for simple designs as well as complicated.
I cannot, then, see how fan art has any bearing on the efficacy of a character design, especially when I consider the Awesome Fan Art threads.