First off, Mabel and Dipper are perfect. They were drawn just like in the show.
Discord, however, is correct. Several times in the show they made a mistake on him. Unlike the animators who barely pay attention to details on a character, you seem to put a lot of time and patience in your work. I also liked how you added the cotton candy clouds above them. Very refrencey.
Overall, it's beautiful. Can't deny it's one of the best Gravity Falls and MLP crossover pictures there are out there.