It was very manipulative, in my view. Elvie seemed like a selfish little twit to me, too busy being curled up in her room feeling morally superior to help around the house or pull her own weight in any way, expecting her mother to take care of her hand and foot... yeah, I hated the movie. This was my interpretation. If you didn't get that out of it, well, that's you. Me, that's what I got.
Actually, that was their form of escapism as they both were non-assertive, Elvie to the point of being an anti-social recluse. As such it was merely benign, but also a bit pathetic.
Elvie was a head case throughout the movie. Only in the end does she take the first steps toward normal human interactions. I didn't see any deeper meanings in the theme other than a vehicle to showcase the star's singing talent amid some actors portraying uncaring or exploitive characters.