While I do admit the first 15-ish minutes were not that engaging to me, feeling like another standard kids movie, it really started to pick up a little after Whitebelly (the fox), and Wheezy (the mouse) meet.
First of all the movie has some of the best looking stop motion I've seen in quite a while, visually looking like The Fantastic Mr. Fox, and possibly a bit of Isle of Dogs. Even though most of it is physical props in stop motion, the rare thing that's not actually stop motion such as large pools of water, and the whale towards the end mesh well together despite being clearly CGI.
The character of Wheezy (mouse) was initially a little two-dimensional feeling with her personality, she and the other personalities got a bit better and more immersive feeling, while the character of Whitebelly and the supporting cast felt pretty strong (except for the Moles, they felt a bit like a walking gag).
The ending, and the whole message of the movie itself is pretty impactful.
I'd go out of your way to find this if possible, just try to stick with it past the first 15 or so minutes and you'll find something pretty underrated.
I watched this on the English dub, and not the original French one as I don't like subtitle blindness, nor can I speak French. *shrug*