It’s an edge-case for sure. There is nothing wrong with having hairy pits in public, in general. As others have said, it’s completely normal for students to wear pajamas to most lectures. And if you had worn it to a large lecture hall with ~200 other students, probably no one would notice or care. Vs. wearing swimwear or something which would definitely seem odd.

However, this particular professor noticed it and said something, so unless you want to battle with them, probably better to just respect their wishes and not wear one.

The interesting thing with the other comment comparing it to a woman wearing a swimsuit with pubic hair showing: first of all I don’t consider armpit hair to be in the same category as pubic hair, and not the same thing if it shows in public. Even so, there is nothing illegal about having visible pubic hair showing outside of a swimsuit, but it does defy a social expectation of female beauty standards, just like visible armpit hair of any kind. So for someone to show either risks getting pushback at the beach for how people ought to look, even if it isn’t anyone’s business. Because it also extends to “girls shouldn’t dye their hair green,” “girls shouldn’t have short hair,” etc., it’s all social expectations and conformity, just to different degrees.

For you as a male, visible armpit hair might defy some people’s expectations as what is “polite” to show in a serious environment like college. Some other professors might not like pajamas either, but they know that ship has sailed. It is up to you how much you want to self-censor to “go a long to get along” vs show up how you want to show up and push the boundaries of society.