But fairytales weren't meant to be pretty. As oral tradition they were morality tales that impinged on the Id, designed to scare kids off from, figuratively, "going into the dark woods at night". The Brothers Grimm were grim. Hans Christian Anderson's fairy tales have a tragic edge, as did the man himself. So demanding a "pretty" Rusalka is like howling at the moon. A Rusalka's beauty was illusion designed to trap the unwary.
Rusalka are associated with water which gives life but can also mean death. While the mermaids of the ocean have the freedom of the seas, water spirits in inland Europe are often associated with brackish water and dense, menacing vegetation. One hardly needs Freud to link water with hidden desires and danger. Anyone who listens to Schubert will know what babbling brooks can mean. Throughout Europe water spirits are a metaphor. Think Mörike's Nixe Binsenfuss, set by Hugo Wolf, who guards the fish from predatory fishermen, and of course, The Lorelei, who sing on the Rhine. And further back in time, the Sirens oif Greek mythology.
Rusalki and water spirits are often associated with sex, which is why they present as beauties. Dvořák's Rusalka will sacrifice all for love.which makes her unusual for her kind, and more "human" to ours. So Dvořák makes us fall in love with Rusalka's beautiful music. Even the Prince learns that death is better than living without love. But let's not forget, Rusalka is a tragedy, and "deep" in many ways.