Twilight Sparkle imitating her favorite pony, Starswirl the Bearded. She’s basing that off of that time he told the most evil pony in all of the universe that she’s 100% evil for trying to destroy both universes. Twilight heard about this story from her evil mentor and is able to perfectly replicate it and tell her own teacher what an awful, awful pony she is. Celestia always pretends to cry for sympathy whenever Twi does this impression, but it’s worth it to remind her what bad pony she is. :3
 
(I like Celestia as a character, but she is evil. Just like how I like Emperor Palpatine, but I don’t think blowing up planets is good. Celestia is a good character, but an evil pony.)