Ever since Luna began her hobby of sneaking into the dreams of children and playing supernatural psychologist, people have been writing stories of Luna appearing in dreams of pretty much every character in the show, either to help them deal with issues or just mess with them. With Diamond, these types of stories generally amount to helping her deal with the “reason she’s a bully”. Sometimes that involves overcoming some fear, sometimes it involves helping her cope with a troublesome home life, and sometimes it involves a straight-up lecture.
 
Oddly enough, I’ve never actually read a comedy involving Luna entering Diamond’s dream. Any time the two meet, it seems to immediately create a vortex of pure drama. If Luna were to appear in Diamond’s dream in the show, I’d imagine Diamond would at least be excited to have a princess in her dream, that is, unless Diamond dreams about the princesses all the time.