oh i mean i've definitely seen discussion of this in the fandom before, if mostly somewhat flippant references to how bad that was. yes, phrenology has been used as a tool of racist oppression in far more places than just sweden. it's possible that it's less known in other places, but i'm american and was able to recognize it. my inclination is to believe that the scene with professor zei measuring aang's skull was written thoughtlessly, by someone who had seen references to skull-measuring at some point but didn't actually know what it implied. it's possible that they were trying to subtly imply something about zei (maybe that he cares more about knowledge for knowledge's sake than about how that knowledge is applied and who it harms). that would also go along with the fact that he refers to aang as a "living relic"—a truly insanely offensive thing to say to him—and really does seem to think of him as an object of study rather than a person. he then, of course, ends up being buried in the library, because of the very fact that he values "knowledge" in the abstract without caring about being able to accomplish anything material with it. but again, this may be too generous of a reading; even if this is what the writers were going for, i still think it's pretty distasteful to show a skull-measuring scene completely out-of-the-blue like that in a children's show without properly addressing it. like you said, atla handles heavy topics, but this really only works if the audience recognizes the device for what it is and what real-world history it has, which many of the children for whom the show is intended would not.
