It is canon. 'Tis how she arrived on the first Luna-centric episode (in fact, the first episode where she even appears) since the pilot, "Luna Eclipsed". It's fun to compare her personality in that episode to Celestia. Surprisingly, much of her personality as Nightmare Moon is still retained. Turns out that the Elements of Harmony can only change someone so much. She has the same voice (when she's being loud), has a similar appearance (a mix between her two versions as seen in the pilot), and has a penchant for trying to intimidate her subjects. Turns out she does MEAN well, but lacks the social skills necessary to inspire love and friendship, which she does desire. This actually puts her one OVER Celestia, who is socially skilled and knows how to inspire friendship and love, but (if you notice carefully) has done little-to-nothing to try and alleviate how her subjects anxiety towards approaching her. She holds the Gala every year despite believing it's a terrible event that bores everyone, she never tries to tell anyone who falls over themselves trying to please her that they shouldn't be so worried about it. In fact, there's one part of the episode where the public reaction to Luna is not so different from that of their reaction to Celestia, but Luna actually is worried about how people kowtow to her. She is, in fact, a slightly more moral person than her sister but lacks the skills to show it.
Eh, I was hoping during the entire episode surrounding Discord that the two would have a troll-off. Mind you, Celestia put the frozen Discord, who could only be awoken by the fighting of friends, in a public place as opposed to a deep dark dungeon without a door. If THAT'S not trolling I don't really know what is.
So, hold on, is this from an actual spoiler image? If so, then I think people need to calm down and think back to ANOTHER pony who wore a cloak in season 1 and was judged because she looked mysteriously evil XD