The novelization and TV movie adaptation aside, Bobbie Gentry has always said her song, "Ode to Billie Joe" was about "indifference" and the matter-of-fact way the family discussed what had happened. In addition to the many times she's addressed the subject publicly, one of my favorite examples is from this exhaustive page about the song, which quotes the screenplay's author as saying:
Max and I go to meet her, and I ask her what does the song mean?
She said, ‘I made it up. I don’t know what it means.’
I said, ‘You don’t know why he jumped off the bridge?’
She said, ‘I have no idea.’
BTW: "Max" was the producer of the film, Max Baer Jr., whom most people would recognize as his most famous character, "Jethro Bodine".
In contrast to the sparse staging of Ms. Gentry's performance of "Billie Joe" on The Smothers Brothers Show, here's the video for Reba McEntire's cover of her "Fancy".