While I haven’t read many of the Mary Poppins books written by P.L. Travers, I have read the first, and I can certainly concur that in that book Poppins is a bit more surly than Julie Andrews is in the film.
While that may be the intention in the books as well, it doesn’t come across quite as much.
While the actress’ performance certainly portrays Mary Poppins as somebody who isn’t going to explain herself to anybody, and who can be quite forceful when she wants to be, you always get the impression in the film that it’s all done with love.
You never get to see Mary’s face and know if she’s smirking or not.