I've just started a new game in Sunset Valley with a single sim, and it looks like sims that are supposed to be dead just spawned all around the town, and not as ghosts.

Almost all graves have been changed to remains, as the sims have somehow been resurrected in the very beginning of the game.

Has anyone seen anything like this before? It's kinda creeping me out.

It has happened to me when I exited to the main menu and went to another world, without exiting all the way to the desktop. I don't know if anything else can cause this, but not exiting all the way to the desktop is a known cause. If you quite without saving, or you have an earlier save to revert to, you won't have a lasting problem, otherwise the world is probably toast.

That is exactly what I had done just before, I was in another game and I went to the main menu and started a new game. But I did save and quit on this file and as I started it up again just now, they're all gone and back in their graves again.

Strange.