Not a lotta effort put into this one since it's so late.
In the season 2 episode "Luna Eclipsed," I was a little disappointed to find out no one guessed Twilight was dressed as The Great and Powerful Trixie. Out of all the guesses they made, I thought Trixie would be the most obvious.
So, as a joke, I dressed Trixie (pony on the left. Yes, that's her.) up in what I thought she thought an appropriate Twilight Sparkle costume would be, in hopes of making Twilight look ridiculous. Little did she know Twilight had done the same thing for her (not really, but she didn't know that. )
And I tell you all this so that maybe you will get the joke.
...Which probably means it wasn't the best joke to begin with...
I was pretty sure she was done up as Starswirl the Bearded, actually... thus, the beard. Though Trixie would have been pretty hilarious, as your picture proves handily!
Trixie: Dur hur, I'm Twilight! let's do friendly tea parties and shop for cute hats with FRIENDSHIP! My parents were so girly, they named me after a vampire!