Lappajärvi Museum is a small local-history stop rather than a major regional museum. Its value is in grounding the lake and crater story in the ordinary lives of the people who farmed, fished, built, and travelled around this part of South Ostrobothnia.
It is worth adding if you have time in the village, especially on a rainy day or if you prefer local texture to only scenic stops. Keep expectations modest and you will get a clearer sense of the community behind the map name.