I think the film (largely thanks to the biography that was also a major source) made an honest attempt to put her attiude in the proper perspective of her personal history. A courtesy not always offered to Persons Of Attitude.
I've studied what history exists to know that much of the film was as accurate as might be, considering the constraints and demands of the media. I'm also aware of what sections were fabricated or modified for theatrical purposes, and I don't think they did any serious disservice to the memories of either Travers or Disney.
Biographical histories are always going to be difficult, just as it's difficult to understand the motives of any historical person, recent or ancient. I think this film did a good job of staying true to the facts and personalities, while also telling a good enough story to make it watchable.
I saw "Mary Poppins" on its first release. I was awed. Today, I'm still impressed by the technologies that were introduced in that film, and by some outstanding performances, particularly Andrews and Tomlinson. Time has made more obvious the flaws, or at least the primitive nature of some of the effects and technologies, but that awareness does not detract from the story. Today, with "Saving Mr. Banks" and the publications it was inspired by and which it generated, we can see "Mary" in a new light, one that adds some depth to the original story, and more than a bit of poignancy.
Was it an accurate biography? Of course not. Films have their own internal logic, and their own commercial requirements, and unlike real life, must be logical. I think better questions are: Was it an honest biography? and Was it a good and worthy story?
For me, the answer is yes on both counts.

Of course, Travers would have hated this film too, but perhaps now we can better understand why.