The real issue with the cat returns is that it's too short. With a runtime of only 75 minutes, by the time the plot starts moving its already half way through. On top of that, the movie has a really anti climatic climax with some of the most boring and underdeveloped characters I've seen in a Ghibli film. (and if you've seen a lot of Ghibli films, you know that's saying something) With all that being said, there is a lot of stuff I still enjoy. Like (most) other Ghibli movies, the animation is fantastic and very imaginative, and while the sound track wasn't as good as other Ghibli movies, it was still very good.
Overall I think this movie is wasted potential. I think if it were longer than 75 minutes, it could have explored its world a bit more and have better pacing. Even with fantastic animation and sound, that couldn't save it. So that's it, see it if you want.