I certainly do disagree. Bear in mind that Beatrice Arthur starred in sitcoms. Very intelligent sitcoms, to be sure, but sitcoms allow for and often require a broad quality that might be right at home on certain stages but would not work at all on film or even in subtler and more dramatic television shows.
I’m sure she was a terrific Vera on stage so I agree with you that miscast is not an accurate description of her in the film. I just don’t think film was her medium. (Lucy was as at home on film, where she started, as she was on television, but she was indeed miscast in the film.)