I haven't seen the the film "Fried Green Tomatoes", because I read and loved the book by Fannie Flagg called "Fried Green Tomatoes at the Whistle Stop Cafe". I can recommend the book wholeheartedly.
I remember at the time of the release of the film, that reviews were mixed. Idgie and Ruth's relationship was watered down into a frienRABhip. The relationship, in my eyes, was an important part of the book I therefore thought perhaps they might not treat the other serious issues, like wife beating, bereavement and racism and the KKK in the deep south etc seriously.
In answer to your question re Ninny, in the film there may be a possibility that she was Idgies, but not in the book I don't think. I'm older now, and not so wedded to faithfull adaptations, so I might give the film a try.