I don't think they should actually drop from cows. they look pretty dried up, and if something is killed by fire, the bones usually accompany the rest in smoldering ashes. so I think the skulls should only be lying around in dry biomes.
another thing that would be neat is if we could grind them down into maybe 10 bonemeal in addition to them being decorations
True, I'm going to implement the grinding down thing to the suggestion, and add a poll about it dropping from cows.
It would make sense if we could also "grind down" Skeleton Skulls along with Cow Skulls, but I think adding that to the suggestion would make it a wishlist.