For me, Too Many Pinkie Pies was an episode that I really wanted to enjoy given its premise . Pinkie Pie making clones of herself), but in the end is disappointing due to writer not taking full advantage of the concept.
The first act (and the first minute of the second act) actually does a great job with setting up what could be a great premise: Pinkie Pie, being the friendly pony that she is, wants to spend time with all of her friends. Thus, when two friends are doing something different at the same time (swimming with Rainbow Dash and raising a barn with Applejack), she decides that rather than choosing one or the other, she instead uses a magical pond to make a clone of herself in order to attend both activities. The first act also sets up a good reason for why Pinkie would make additional clones, as one of the clones discovers a third activity that she's interested in attending (a picnic with Fluttershy and her animal pals).
Unfortunately, the problems with the episode not reaching its full potential start showing up when the Pinkie clone learns about the cloning pond herself. The first problem is that rather than slowly building things up to result in Pinkie losing track of how many clones she made (and having the other ponies outside of Rainbow Dash notice more than one Pinkie Pie), the episode pretty much rushes into a mass amount of Pinkie Pie clones.
The second problem is that, outside of the bit to determine which Pinkie Pie is the real one (which itself is somewhat of an issue with the episode, as the real Pinkie Pie in this episode shows she's clearly to show different emotions, while everyone else can only think about having fun), the episode doesn't really take advantage of doing something truly hilarious with a whole bunch of Pinkie Pies (when compared to the Treehouse of Horror story on the Simpsons where Homer made clones of himself). Instead, most of the antics of the Pinkie clones consist of them being annoying while recycling animation from previous episodes.
I'm sure a storyline where Pinkie Pie makes clones of herself can indeed result in a huge laugh riot. However, this episode, by rushing the mass amount of Pinkies and not taking full advantage of the situation, just doesn't do it for me.