From the Super Mario Wiki
In the episode, when the Koopa Packs have Mario and Luigi surrounded, Princess Toadstool gets a Starman and becomes Super Princess. The color of her dress changes to blue and white, and Super Princess becomes more powerful (though she does not shoot fireballs). She was able to defeat the Koopas single-handedly.
The Princess says "Princess P. to the rescue!", after getting the star, which is a nod to the name "Peach", which had only been the Princess's name in the Japanese versions of the games at the time.