Well I very recently just beat the endgame of mistlands in a run without using vegvisirs to find bosses. I can assure you it wasnt really that hard to find skulls when I was finecombing mistlands to find that boss altar.

About every single ribbcage i found, also had a skull. You do know there is a possibility that skull/ribbcages spawn completely upside down? The lowest ribb pointing straight upward making skull sometimes buried, sometimes skull is molded into unbreakable root or into a mountainside.