Well, no, but animating against a prerecorded voice actor is. I think we just have to accept for now that cartoon characters lipsync. In the future of course, we should expect deepfake-style networks should be able to produce both the voice and the mouthpart motions from a single script.
Only when you're dubbing over foreign cartoons.
In most other situations, when you are making the cartoon for the first time, the voices are recorded BEFORE the animation is finished so that it's easier to animate the lip flaps.