Originally posted by Jolie Rouge:

There's text written on some stones, sometimes you get a hint on what happened and how the buildings came to ruins, what's the story with the Draugr and so on.

Originally posted by pipo.p:

Their graveyards, and all their dead, have not to date back to the very day when the Gods destroyed them, and there were nobody left to burry them! Undeads, on the other hand...

Let's recap.

• Valheim was populated by a civilization that erected all the crypts, burrial chambers, graves of any kind, perhaps also any non-wooden abandoned structures (e.g. stone towers).
• The Gods destroyed these people, some of them being turned into undead (skelettons, draugrs).
• The depleted Tenth World was then used as the last home for depraved warriors' souls, who didn't earn a ticket to Hell (greydwarves), being like a lesser hell for them.
• Odin banished a few demons there as well, or primeval creatures having became too dangerous on Middle Earth or other worlds. Then the Tenth World was cut loose and left alone.
• Portents are bad, as it seems that the Forsaken have recovered from their initial banishment, shared Valheim between them, and are perhaps even able to return on the Tree and be a nuisance again.
• Odin tasks Valkyries to bring brave, yet not totally pure, warriors to Valheim, both as a personal trial purgatory, and as a mean to lessen the Forsaken by killing their avatars again and again.
• Vikings in Valheim are immortal until they earn access to Valhalla, and as such they never build graves for their buddies. Nonetheless, they are probably the builders of recently abandonned wooden structures, because such buildings are not to withstand Valheim's weather for long.

For now, the Stones don't tell if lazy vikings are cursed to become Greydwarves, or if they just stand in Valheim forever.

TL;dR: I don't see any major plot hole in Valheim's storyline.