This one's on me, because I was trying to make a point and be sarcastic at the same time.

While I agree with you, I'll quibble that context isn't actually important here. The point of the shirt, and the point I was trying to make, was that reducing an entire culture to a caricature is offensive in any context (regardless of whether or not you would it offensive). The people who defend using native imagery for sports mascots often claim that the portrayal is intended as an honor, and that they are celebrating the culture instead of demeaning it.

All of your points are correct, and each one compounds the offense. But even if you turn it around and use white people as the mascot, a people and culture who have not been systematically oppressed, deprived of life, liberty, and property, it's still an inherently prejudicial and ignorant thing to do. So any argument about how mascots are intended to promote or celebrate or honor a culture or a people still fails to justify the practice.