“I know you, I walked with you once upon a dream,” she sings, a cappella for a few lines, in a slowed-way-down version of Princess Aurora and Prince Philip’s theme song from the 1959 animated classic Sleeping Beauty. Del Rey’s timbre instantly evokes the atmosphere of her oeuvre. More than a hazy, Instagram-y dream world, it’s a signature dissociative state—being sad but feeling beautiful, observing one’s own desolation through the lens of a tingling body high. Fairy tales are set in a mythic realm of feudalism and candelabras, but, of course, they’re written and rewritten to reflect contemporary anxieties and aspirations. Who better than Del Rey—with her thing for tragic archetypes, her widespread castigation as a fraud, and her rise to stardom despite it—to bring Maleficent’s excavation of female evil, its themes of cruelty, enchantment and transformation, into the present?...And when the credits roll, it’s a haunting postscript to the Disney resolution. Del Rey represents the real possibility of an unhappy ending " - Johanna Fateman, The New Inquiry

It was an interesting choice for the film to attempt to reassign Princess Aurora's love song to Prince Philip to the villain of her story, who suffers from a lack of love and has her fair share of evil intentions. However, without changing any of the lyrics, Lana Del Rey breathes new "beautiful, tragic, and melancholic" air into the song that makes the song Maleficent's. There are many ways to interpret it in the context of the film, whether it be Maleficent reflecting on what could've been with her young love Stefan, that ended in betrayal and punishment, or her moral transformation after reuniting with an older Aurora. As Fateman describes, Lana's voice often is "sad and beautiful", which I think reflects the personality and trauma of Maleficent's character  - at least in the 2014 version of her being played by the gorgeous Angelina Jolie. 

Source:

Fateman, J. (updated 2017, April 18). Once upon a dream. The New Inquiry. https://thenewinquiry.com/once-upon-a-dream/