Martin created a literally superior race (can handle dragons) who are as white as it gets (even the violet eyes are probably a reference to the red eyes of albinos), who ruled Westeros, ruled important parts of Essos, were some kind of Atlantic super advanced civilization, and are now going to save the world (Jon after all is at least a half-member of that superior race).
I think it is hard to deny the tone of white supremacy in that. Or in Lord of the Rings.
Lacking diversity is one thing. Featuring supremacy another.
However, this is IMO still, most of all, incidental racism. What I mean by that is that the story elements as such, the superior ancient race, the primitive barbarians, the foreign allies of an evil force etc etc, aren't racist in and of themselves. They are just story elements of a sort of natural attraction within the fantasy genre. Each one of those has IMO something going for it in an entirely harmless entertainment-wise way which just lends itself to be being used, So it is used. And since, as others already explained in length, it is often used by writers who can best identify with a white European heritage, it will be used in ways that are incidentally racist. The superior race will be white, the barbarians will be of darker skin tone etc.
I am fine with making people aware of that. Won't hurt, is an IMO interesting observation, may do some good. But as innonamtu already kinda said, I do not want my escapist fiction to police me or be policed. I want it to intrigue me, that benefits from a writer going with instinct and heart. So I don't really care about incidental racism as long as it is not intentional racism, as long as it is not policing me in one way or the other. Though if it finds a good way to do so, it can always educate me. And if it finds a good way to be politically correct, I am fine with that, as well. But as soon as it feels forced, it is a way to damage my immersion, and that is exactly the kind of thing I do not want from my escapist entertainment. And perfect diversity will tend to feel forced.
On the other hand, cultural, nations, ethnic groups and ways of life beating the crap out of each other feels quit natural.