Are Yamask actually the souls of dead humans, as the Pokédex states? I feel a bit odd about catching and training Yamask because of it. Sure, it’s not the worst afterlife fate I could imagine, but it still weirds me out.

Luckily, no. Just like phantump, this is only Unovan folklore, albeit a popular story. It stems from the fact that people have a tendency to look for something familiar in the unfamiliar, especially when it comes to things that might vaguely resemble faces. So while yamask’s mask is generically humanlike, it’s only a generic humanlike mask, created to help yamask harvest negative emotion for its meals. If you stare at it long enough, you may begin to make out the face of someone you know who has died, but this is just your mind using your memory of them to play tricks on you.

In truth, yamask are just like any other pokémon: they hatch from eggs, no death necessary.