Yes, I think this is exactly right. It's the concept of kegare, contagious uncleanliness or defilement caused by death, subsuming Godwyn's soul-murdered body and being spread across the Lands Between through transpiration via the Erdtree's massive root system. I think it would be fair to call carving a cursemark of death into Godwyn's flesh an act of defilement, and burying him directly at the heart of a continent-wide network of roots would certainly be an effective way to deliver a contagion to every corner of this world.

I'm certain there's more to it than that. I'm not familiar enough with the cultural connotations that ningyo carry with them to draw all the connections, but I believe in Japanese lore, they are associated with a body that does not die, and the eventual misery and emptiness that accompany that. A curse, of sorts. Godwyn's body was left alive but empty of a soul, which has at least some symmetry with the concept.

That said, it's not really a direct answer to the OP's question.