Did you notice how, when she was talking, she kept switching between different tones and emotions? Sarcastic, amused, then angry. 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.
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, I mean, 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.
And I love how she's supposedly a wise loving mentor but fucked up by ignoring the warning that was presumably meant for her in the first place. I mean, wow, this show actually has surprising moral ambiguity! The main character is a misanthropic (misequinic?) nerd and the other ponies are nice while still being morons. They're all legitimately flawed characters.