Classic French animation. Sometimes cited as one of the best animated film ever produced. I don't have a source for that quote so I'm going to assume it was made a long (long) time ago, or by someone who has little affinity with the genre, but it's true that the film manages to stand out, even today.
The King and the Mockingbird offers an interesting variation on the famous story by Hans Christian Andersen. The premise has remained largely the same, but some retro sci-fi elements add an extra level of intrigue not present in the original story. It retains its fairy tale-like qualities though, meaning it does feel a little shallow.
The animation is pretty delicate, the art style quite distinctive. There are minor Disney influences that detract a little, but they are few and far between. Not everything is equally interesting and the film has some ups and down, but for a 40-year-old animation there are still a good few scenes that stand out. Nice, though a tad overrated.