When Jim Carrey is impersonating, it's clear that it's Jim Carrey impersonating someone for comedy-sake, not providing a service in lieu of someone else. In other words, Jim Carrey isn't getting paid to stand in for Jack Nicholson for example. Otherwise, it looks more like the Midler vs Ford Motor Co. case[1]
Are you referring to doing impressions where the act lasts for a few minutes, or are you saying that Jim Carrey actually impersonated other celebrity voice for like a whole movie or interview? There is a difference, I think. One feels like “fair use” while the other would seem more like “plagiarism”.
As pointed out upthread, fair use is an exemption for copyright. You don’t need fair use for something that isn’t copyrighted (and, indeed, isn’t even copyrightable).
