No it's not. That little dream bubble in A Royal Problem just had Rainbow and Twilight flying together. That does not mean Pony Girl is canon. They flew next to each other like that back in "Testing Testing 1, 2, 3", so it could easily be a callback to that. Or it's just a random dream they decided to make about Twi and Dash hanging out, and it's not a callback or reference to anything. Either way, I do no think it's a reference to Pony Girl. A Royal Problem was released early in Canada less than a month after the release of Pony Girl, and even at the time Pony Girl was released (April 29th 2017) "A Royal Problem" was finished, just not released. The episodes are done many weeks in advance, so it couldn't possibly have been a reference to Pony Girl. Also, why would they reference Pony Girl? There's been so many other fan projects bigger than this in the fandom over the years that's never been referenced in the show, so why Pony Girl? As far as I'm aware, the staff is also not allowed to take anything from the fandom and put it in an episode. (Episode 100 was an exception if I'm not mistaken).