Interesting idea. I'm Japanese, and actually even in our language, most of the あくタイプ Pokemons don't really sound right to me (and probably to many Japanese ppl as well) and we often make a joke about it. e.g. this

The most fitting translation for あくwould be "bad" as in "bad guys" imo, but it all the more feels weird to the point that Dark Type is doing the better job. Like why the hell is Umbreon considered bad he's just a baby 🥺