Twilight seems to be more of a fallback princess, working in the background (background pony har har) or stepping in when nobody else is up to a task. Even her role in this episode was largely ceremonial. As she said, the attending princess had never had to do anything before (and it really sounds like a responsibility that could be delegated to some sort of magistrate). She's still receiving instructional material from Celestia, and she doesn't have a throne of her own.
Twilight has made a conscious effort to avoid the spotlight and I new wouldn't be surprised if Celestia is trying to keep her out of it as well. Ponies need to know she's a princess, sure, and her coronation was public, but too much exposure and they'll be beating down her door (Twilight Time?). Baby steps to get her accustomed to a role of authority and requiring heavy use of "people skills," things that naturally clash with her personality. Look how much trouble it was just for her to interact in a relaxed manner with the others once she arrived in Ponyville. Twilight never asked for this. Laying low and keeping her responsibilities small is a good idea for now. Presiding over this event was a good opportunity to begin easing into the public eye.
Sorry if that seems rather disjointed and like I'm rambling. Typing this from my phone at work as I can and it makes things rather difficult in a number of ways. ^^;