Sarcasm is an adjusted stress reaction.

given that Bill's already seen & experienced certified weirdness, and that it was quickly established that she's tying her experiences back to science fiction & knows the genre, it still felt very odd and out of place. like in Rings of Ahkaten, Clara has one adventure under her belt & her experience with aliens is basically "handsome dork with a time machine."

whereas by Oxygen Bill has experienced so much more that she's taken in stride, and on top of that she's very genre savvy. why would she be shocked by a person in space who doesn't look human, except to make a joke about space racism that doesn't even fit the character? so while I'd agree that Clara's big thing is shows of confidence when she's not really that confident, it was still a garbagey writing choice for the relatively more experienced, more genre savvy, and "more likely to have experienced actual racism" to be like "omg blue dude, let me stare and be defensive over accusations of racism."