Double Rainboom is pretty good at what it is--a Hanna-Barbera-style show, using MLP characters flattened down to their basic repertoire of tics (like Hanna-Barbera characters are). It has funny animations and constant action. But it isn't a story. It's two different stories pasted together: a framing story about interactions between Twilight and Rainbow, and a crackfic about Rainbow Dash vs. Powerpuff Girls.
This over-long interlude with the Powerpuff Girls was connected to the framing story only by mere physical causality: events from the framing story caused Rainbow to enter another dimension. But there was no intentional causality. Rainbow went there by random chance, not due to any decision of her own. Her exit from it was also, from her perspective, pure luck. So you can't say Rainbow had a character arc, since the events she supposedly learned to avoid, couldn't have been predicted.
It was completely unsatisfactory to anyone expecting MLP, which was extremely character-based under the blessed rule of Lauren Faust. That doesn't mean just repeating their basic stereotypical kinks. It also doesn't mean putting characters into the most-obvious conflicts their characters could have. A classic MLP G4 episode (meaning seasons 1+2) shows a situation in which the ponies learn how to work together, using their different individual talents as necessary, rather than each trying to do things their own way. Double Rainboom doesn't even have a full conflict, because it never explains what Twilight wants to do with the potion that Rainbow steals. It is physical conflict without character conflict, and physical causality without intentional causality.
