The thing about The Cutie Pox is that it has energy and plenty of fun gags. It's an episode that is, at the very least, fun to watch, but in the end it really has nothing else to offer. Nobody learns anything, nothing important happens, and at the end everything is the same as it was in the beginning. In other words, it's pure filler; an episode with no importance.
Now, the episode isn't entirely worthless. There's a few clever jokes, and there's certainly plenty of on-screen energy. It's definitely a watchable episode, and in some ways it's more enjoyable than the arguably superior Sisterhooves Social. It's not like filler is necessarily a bad thing, after all: It can still be entertaining, it just doesn't have much worth relevant to the show.
And really, that's the problem. It may have been entertaining, but I have more bad things to say about The Cutie Pox than good. It's just plain shallow. Again, nothing really progresses in this episode. It starts in one place, and it ends in the same place, with nobody having learned anything except maybe the audience. But even with that, the lesson isn't really anything new. The Cutie Mark Crusaders have already been told episode after episode to be patient, and thus there is nothing new learned in this episode. There's ultimately no real point.
It's really evidence of what's wrong with these Cutie Mark Crusaders episodes. The characters are thick-headed kids that don't learn anything, and as such each of their episodes (up to this point) has the same lesson. The result is diminishing returns, until their episodes become almost devoid of value. These characters' sheer refusal to be even remotely sensible or learn any single thing in fact gets annoying at times, especially after several episodes which have just been retreads of each other. That's the problem with Apple Bloom here, especially since by now she really ought to know better.
So the lead character is stupid, the plot accomplishes nothing, and the lesson is a retread. If it weren't for the clever gags, this episode wouldn't have much worth at all, and even now it's blatant filler without any real depth to it. Based on evidence, it's a bad episode, yet it's still fairly enjoyable. Call it a guilty pleasure, if you will, but it's certainly watchable. That said, there's far better episodes that are just as or more watchable, so this one falls flat.