I know that in the books she does have more of an edge to her. And that she isn't as sweet as she comes of in the movies.
I do know that the show was trying to be a hybrid of the books and the movie.
I was at the lecture at Barns and Nobel which featured Brown and the guy that wrote the companion book to Mary Poppins. He said that they took elements from the book and had them lead into things that the audience knew and loved from the movies. An example would be Mrs. Correy and the talking shop which is from the books and have something from the movie like Supercal take place there.
The plot line about Mr. Banks almost loosing his job at the bank and all that was a story line that was created just for the musical and wasn't seen in either the books or the movie.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll