The developers have put something in her castle for a reason. They make Ultimecia's castle look that way (the design, the walls, the paintings, etc) because all of them are supposed to represent Ultimecia's personality. But the developers making outside stuff (the star) to achieve the same goal doesn't seem right.

How can a dying star represent her personality? Unless there's something going on. Like it's dying to show that Ultimecia will die later. Destiny.

If a star is dying, it's supposed to look more red-ish and darker. Maybe Lunar Cry boosted the process of this star dying?

A dying star means Ultimecia and her castle were taken place in millions of years later. I don't see any dying star in Squall's present timeline.