I would have liked it more if they used a Name without the word “nightmare”, but after the same formula like “Nightmare moon”, being something negative + something the pony is associated with. Though it is kind of hard to think of such a name for Rarity.
…also, they may not have felt the need to come up with a fancy name just for the benefit of Elements of Harmony. The Nightmare likely would just dispose of Rarity once she outlived her usefulness; in fact, I’m pretty sure the comic pulled a Venom with the Nightmare wanting to return to Luna…