Interesting elements, but flawed characterization.
At first I thought of this as a great episode that I didn't love for some reason, and I didn't know why. On second watch, I see it's because of some issues with the episode. At the same time, what interested me about it before is still interesting now, leaving an episode I have mixed feelings about.
Pinkie Pie's character traits are exaggerated here for the sake of the story. Her obsession with fun is increased, and it actually gets annoying sometimes. I usually love Pinkie Pie, but in this episode she became a bit too much. Her hyper personality comes off as grating here, as her worst excesses are on full display. Sure, Pinkie still manages to be funny quite a few times, particularly when she defies gravity above water, but this is the only time that she's actually bothered me. Her main conflict is something that hasn't popped up before, and it's something that her proper characterization would be too sedate to have to deal with.
Then the clones are introduced, which seem to be constructed only from Pinkie's worst traits, resulting in a torrent of mild irritation. The mirror pool they came from is an interesting piece of lore, but it's one that isn't explored in enough depth to be relevant to much. At the same time, the existence of this piece of lore still expands the fictional universe. The clones' apparent inability to function under stress is interesting, but again, explored in no detail whatsoever.
Meanwhile, however, there's a notable darker element to this episode that manifests in the real Pinkie Pie. She begins to question her existence, clearly being the only one of the Pinkies that can. One must wonder why they don't think that the Pinkie acting different might be the real one, but a character having an existential crisis is still an interesting place for the episode to go. Here, Pinkie fits better with her previous characterization, and is even given some more depth. She develops further as a character, and comes to a point where instead of fun, she simply seeks rest. Pinkie's growing maturity is an interesting character arc that unfortunately hasn't been given the attention it deserves, but when it shines through it enriches the character.
So, the episode's a bit of a mixed bag. It expands the universe and provides a very interesting character moment in Pinkie's existential crisis, but it's also a bit irritating at times and its exploration of the universe is shallow. It's certainly not a bad episode, but it's certainly not my favourite episode of the season.