While Gamefreak Fairies are usually pink and cute, actual myths of fairies describe a lot of them as powerful, mysterious pranksters. The Grimmsnarl line is heavily inspired by Imps. (Impidimp) Wikipedia states that:
An Imp is a European mythological being similar to a fairy or demon, frequently described in folklore and superstition.
So that is why the Grimmsnarl line is Fairy type. Notice how the quote says European. The Galar region is based off the UK, explaining why it's found in Galar. Grimmsnarl also has the ability Prankster, which refrences it's origins.
Also, here's an interesting video on the origins of Grimmsnarl.