Technically Amatsu-Mikaboshi is an especially malevolent Japanese god even in the Marvelverse, not a creature from nothing with bullshit powers. He just got a major powerup out of nowhere due to a long series of circumstances (hint: he planned it all for centuries).
That said, to address KageX: it's also true that Marvel cosmics tend to have powers that transcend creation. Take the Beyonder, who was shitting on everyone and everything pre-retcon, including the Living Tribunal, and threw the entire omniverse in total chaos just because he could and was bored.
While Usagi is immensely powerful, she is so LOCALLY - her powers are never hinted to affect things beyond the universe she resides in. She is nearly omnipotent - relative to her universe. Marvel cosmics, however, tend to be MULTIversal (Galactus has a form of omnipresence, the Living Tribunal flat out IS omnipresent, the Phoenix Force exists as 'shards' across universes, though the Force itself never leaves the White Room at the end of eternity, and transcends the concept of time to boot... the list goes on), so they tend to simply be on a level that Usagi has never found herself anywhere near.
I'm not even sure she could contemplate the omniverse without breaking her mind, since it's basically infinity multiplied infinite times. It's not a concept a limited mind can grasp - one needs to be aware of the omniverse to even begin to understand such a thing.