“Spirit… whose grave is this?”
“Why, YOURS, Snowfall—the richest mare in the cemetery!!!”

@Background Pony Number 17  
Because she kinda is one. She even seems to revel in it to some extent; given what we saw of her in Luna Eclipsed. Plus, even if the Yet-to-Come ghost is meant to be an ominous, foreboding character, it is ultimately a benevolent entity in the story, which fits Luna rather well.

@SaintAbsol  
The interesting thing is the “casting” of the characters in the story-within-a-story was done by Twilight Sparkle. Note that she was doing Pinkie’s “Spirit of Hearth’s Warming Presents” in Pinkie’s voice, indicating that she was basing the characters in the story off of her friends.
 
Which means that, even though Twilight has been friends with Luna for some time, and probably knows her better than any pony but Celestia, Twilight STILL sees her as an ominous, foreboding character.

“I AM VENGEANCE…I AM THE NIGHT…I AM BATMAN”

@Kamazeustra  
Considering the role she was playing in the story, that’s not surprising. The ‘Yet-to-Come’ ghost is meant to be a final warning of the path the main character of the story currently walks; dark, foreboding, and utterly uncompromising in the visions they show.

Only now I notice how Luna’s horn is glowing.

It was kind of funny how harsh this song was. Luna pulled no punches.

 
I still think this was a better ghost of Christmas future.