I think bomb rocks are like those candies that pop in your mouth. They're found inside cans in Pikmin 1, which would imply they're edible in some form. And they're found near skulls in the same game: Perhaps whatever creature that was tried to eat them? The hardened rock-like texture may mean that they've expired.

Obviously the sheer popping power hurts enemies so small, but to a human, they would most likely be fine to eat.

This does call into question why you can make them out of raw material in Pikmin 4, but you can also make scrummy bones, and Oatchi can eat those no problem. So either Oatchi can digest clay, or the raw material is actually edible.