In regards to religion and mythology in the context of this little playworld, I'd have to advise against it. I quote Peter Molyneux on religion: "If I include religion in my games, I'll either offend someone with my ignorance, or insult them with my irreverence." Unless you're fashioning a world personally a la Tolkien in which you purposefully want it dominated by the One God, or a more dualistic setting a la Robert Jordan, its best to just leave it a blank mystery. Seeing as how this is not our world, and many, many of us are screwing around with it, the most agreeable choice to all those involved would be to simply leave it a mystery, don't you think? For that matter I definitely see deific parallels in the idea of Celestia. She even refers to twilight as "her faithful student," which is of course awfully close to the commonly used Christian line "Well done my good and faithful servant." However, seeing as how it is indeed a children's show, I would be willing to surmise that Celestia was simply intended to be magical royalty, and nothing more. We can observe that she displays multiple traits lacking divinity, such as vulnerability, as we saw in Sonic Rainboom where she shrunk away from that cloud splat that hit her throne, and that she could be conquered by her little sister, and a lack of immortality given that her sister is obviously younger than her and can age. It is far more likely that she is just similar to the pegasi in the sense that her job is a control over the astronomical aspects of nature and her access to magic allows her to lift the sun and the moon.