Ahem, let me grab my spectacles and point my finger in the air…
This isn't the first time a Cubone has revealed their face — I can't recall whether it was in Pokespecial or the Pokemon Pocket Monsters Manga, but a Cubone is seen without their skull helmet in one of these series! I'll try and track down the panel and bring it back here…
Here's source on the artwork in the bootleg above:
http://alounea.deviantart.com/art/Cubone-Osselait-434532177
EDIT
Here's some official art from way back showing most of Cubones face
The panel from the manga is still out there somewhere… I'll find it!