I don't think this exemption is fine, because it turns an entire culture into a mascot and a generalization. I think it's one thing to depict a person, but there is a difference between depiction of people and perpetuating a stereotype to the point of canonizing it. It turns Native peoples to a fictional character.
I'm not sure I agree that Scafidi would exempt putting a Native American man with a headdress on a Western styled sweater. Is it really that much different from Chief Wahoo or the Redskins or even my beloved Braves? I don't think so, and don't think that Scafidi's original 3 "S's" really apply to this, as that was more for a culture's objects and creations.
I mean, yes, isn't it obvious that the critique is directed towards the designer (Ralph, Hiroki) and not the actual cartoon?