There is already an official Christmas Tree ornament, made available by American Greetings. Unfortunately, that’s simply not enough to completely ponify your tree. Oona pointed out Southparktaoist’s christmas ornament project, based on classic Christmas Themes. As you can tell with the ghosts of Past, Present and Future.
He makes the images and is working on putting them to PDF with cutout lines. You print them out and glue them onto a blank ornament, easy as that.
Pinkie Pie the Red Nose Reindeer: [link]
Twilight Sparkle as Santa Claus: [link]
Rarity dressed in Winter Fashion: [link]
Rainbow Dash Ghost of Xmas Future: [link] (Note: Rainbow looks 20% too scary so I will make a much more cheerful second ornament for her in the second set)
Fluttershy Ghost of Christmas Past: [link]
Applejack Ghost of Christmas Present: Coming Soon