Did you notice how, when she was talking, she kept switching between different tones and emotions? Sarcastic, amused, then sort of pissed off. And she had this weird movement like she was trying to be regal and dignified, but then the next moment she was zipping back and forth and touching people. Also the maniacal laughter. I think she's nuts. Maybe multiple personalities?
She was probably more sane before she was imprisoned for a thousand years; in the flashback she looked more subdued, though that may have just been the style. Still, what the fuck Princess Celestia? Your sister throws a snit about not being appreciated, which she actually really isn't, so you IMPRISON HER FOR ALL ETERNITY? And since you've clearly not given her a moment's thought ever since, she's been alone all that time? She's doing well not to be a complete lunatic. Ha ha, see what I did there? So Princess Celestia, it's really all your fault. I mean, if you were strong enough to imprison Nightmare Moon, why not just overpower her magic to make the sun come up anyway?
I love it.
tl;dr: OMG moral mabiguity in my little pony