It's like the Ugly Duckling, except with momentary violence in place of a lifetime of anger and resentment.

Feebas evolves into Milotic by maxing out its beauty stat, which implies that, even if you're ugly now, you can become beautiful through hard work and dedication. That sounds about right to me.

Or I guess nowadays you just trade it with the Prism Scale or whatever, but that doesn't sound as nice.