It's fairly common for a close-knit group composed of various races to mix light-hearted jabs like that into everyday conversation. When I was in Korea some of my Asian friends there would jokingly call me round eyes, basically the reverse equivalent of slant eyes. So it is the context in which something like that is said which would determine whether someone finds it offensive.
In the case of Kas, he is just light-hearted kind of guy presumably from Russia. So I wouldn't blame him for the occasional faux pas.
