Does the bold part mean “let’s hope she did not mean to look prim but, anyway, she looked like that”?
Did I understand it correctly?
“It’s all right for you,” said Shasta to Aravis who was walking beside Hwin. “You’ve got shoes on.”
Aravis said nothing and looked prim. Let’s hope she didn’t mean to, but she did.
“us” refers to the writer and the readers of the book. The writer of the book is talking to the readers. he says: let’s hope she did not mean to look prim, but , she looked prim.
I found the quote, and the two sentences are a paragraph by themselves. It sounds like the author is speaking directly to the reader, and that @john1230’s interpretation is probably correct. I suppose it’s possible the intent was “Let’s hope she didn’t mean to say nothing and look prim…”