First off, switching to Cycles is probably quite a bit of work. While the renderers are supposed to be interchangeable, since AFAIK Cycles supports more features than Evee, options that previously did not matter with Evee rendering now have to be set for Cycles.
Also, having seen the film, I found the "unrealistic", cartoonish look very much to be a creative choice. Evee can produce much more "realistic" renders than what you see in the movie, but this requires also much more investment into things like assets and textures, otherwise you quickly land in the uncanny valley. So I think switching to Cycles probably would not matter much, unless the creators would also change their creative choices, which would result in a different movie, but not necessarily a better one.
What's odd in Flow is the contrast between the near-realistic non-animal rendering, and the non-realistic animal rendering. It didn't bother me much - it was clearly an aesthetic choice - but I know people who were bothered by the contrast.
Also, having seen the film, I found the "unrealistic", cartoonish look very much to be a creative choice. Evee can produce much more "realistic" renders than what you see in the movie, but this requires also much more investment into things like assets and textures, otherwise you quickly land in the uncanny valley. So I think switching to Cycles probably would not matter much, unless the creators would also change their creative choices, which would result in a different movie, but not necessarily a better one.