Again, that was the point.
I really wish they kept a longer cut of the movie. We lost the very important sequence with Kermit and Piggy's blow up, and that would have given the film some perspective.
Believing in a dream is one thing, maintaining it is another. Kermit lost his confidence when Piggy split, and Piggy was doing well on the outside without Kermit, but desperately wanted him back. The gang fell apart after that, and they had to find jobs of varying degrees of "well off" and "miserable." Had the film a happy tone throughout, it wouldn't have that extra punch.