It fit the theme, which was Heavenly Bodies: Fashion & The Catholic Imagination. It’s not just a ballgown with a head piece, aka Blake NotSoLively. That said, I think Rihanna’s outfit must have been annoying to sit on with all those pearls and beads. I would have been uncomfortable.
I think the best of the night was Zendaya’s Joan of Arc look:

I think my favorite dress in the “ball gown” category from the Met was Emilia Clarke’s Dolce & Gabbana dress:
