The constellations that comprise her body represent interpersonal bonds – each star is a friend, and so the more friends she has, the more powerful she becomes. The ram horns were inspired by one of the ancient Greek Titans, though I can’t recall his name.
Pinkie: “Ooh! Looks like it’s time for an ascension celebration! I’ll get the pentagram-shaped balloons!”
Rarity: “Oh my word! Darling! Have you outgrown your entire wardrobe? This cannot stand! Spike! Get my measuring tape!”
Rainbow: “So… you’re made of sky now? …Can I fly through you?”
Applejack: “Huh. Y’know, some days Ah think Ah’ve seen it all. Then Ah see somethin’ like this an’ remember that Ah live in Ponyville.”
Fluttershy: Friend? Animal? Monster? Monstrous animal friend? Fluttershy.exe rebooting…
Spike: “So now that you have these phenomenal cosmic powers, can I maybe do fewer chores? Have a little more free time? No? Yeah, I didn’t think so…”
Edited