To me, MMC as a stand-alone episode is fine, just a bit of a pacing problem. It’s only a problem when the whole series is taken into account.
Thing is, it really doesn’t make any sense by itself. There’s a lot of mentions to events to previous events in the series.
Admittedly, Twilight’s transformation wasn’t explained to any extent any point and totally came out of left field, and she’s already proven worthiness in much more impressive fashions earlier in the show. Like in the pilot of the series, just as an example.
Okay, the first part of
Magical Mystery Cure could have stood as a full episode, but the second part desperately needed another episode or two to explain
what the hell was going on.
@Trickquestion
With regards to Rainbow’s mark, they seemed to be switching jobs/lives more than just cutie marks. If they were just trying to live up to each others’ talents, they wouldn’t have swapped houses, after all.
And Applejack should have been digging for jewels if they were going for cutie mark talents.