When I first stepped into Kitakami, I saw a Yanma flying around and immediately went "yes, this is a Japanese country town" before being confused at that initial thought. Dragonflies aren't uniquely Japanese, yet they make a place feel more authentically Japanese? How does that work?

But because Yanma came out in gen 2, a distinctly Japanese generation, it gives it that vibe. It's Game Freak utilising nostalgia to sell a location more strongly.

Spinarak, Wooper, Hoothoot and Bellsprout all help to sell this feeling, even especially Bellsprout's importance to gen 2, despite being a gen 1 Pokemon.

But even the non-Japanese based gens feel connected, or at least, not out of place. The Swaddle line, Cutiefly, Fomantis and Petilil are so small and genetic that their aesthetics don't clash with the strong Japanese. If something like, say, Scraggy was in this DLC, it would have been harder to sell, as it's design is so not lazy country town but rather big bustling city.

The only Pokemon I feel are out of place in this DLC are the Timburr line and the Jangmo-o line, but Jangmo-o was the last pseudo-legendary left to add so it needed to be put somewhere. And I suppose Tumburr are more people-focused, so having them in a wild area terrarium for natural habitat study in Indigo Disk would make less sense.

Tl;dr - Nintendo did a good job at using nostalgia for older Japanese based regions to sell the feeling of a Japanese town.