Why are some shiny Pokémon SO utterly hideous??

I've been shiny hunting lately and decided to try for a shiny Flutter Mane - but I made the mistake of not looking it up beforehand...

Having a shiny be 0.01% of a shade different is one thing (Garchomp, Blissey, Pikachu, etc) but why do so many shiny Pokémon literally look like they're coated in urine??

Look at this hideous thing!! 🤢🤮