What would happen if shows like South Park would remove their introductory disclaimer like message like 'all celebrities are impersonated...poorly'etc. before the show stars. What could happen to them legally? Would they be allowed to and how bad would it be?
That disclaimer -- at least the real version you usually see at the end of movies -- is called the all persons fictitious disclaimer. (The one that South Park uses is slightly different, see below.)
The purpose of such a disclaimer is to attempt to minimize the possibility of a libel suit, if someone believed that the show was using their name, likeness, or personal life as part of the show, without their permission. The benefits of the disclaimer are twofold:
- It asserts that, as far as the production company is aware, everything that happened in the movie was fictional, and was created out of the imagination of the writers, directors, etc. This helps them protect their copyright claims on parts of the story that would otherwise be ineligible for copyright -- such as real-life events.
- It informs the audience that anything they saw in the movie that resembled a real person was unintentional. This is intended to help eliminate anyone being able to claim that the movie damaged their reputation (one of the requirements for a libel suit) by insisting that no one who saw the movie would have believed it to be about the real person.
In practice, the lack of disclaimer, by itself, isn't likely to cause problems for a movie, if the movie wasn't doing anything libelous in the first place. Similarly, if a movie really is using a real person's name/likeness/etc, having the disclaimer isn't going to magically make it OK. However, by making the claim that any such resemblance is unintentional, it shifts the burden on the victim to prove that the studio did something wrong on purpose, and that it had real consequences for them.
In the case of South Park, the disclaimer is just part of the joke. Instead of denying that they are mocking any real people, they're openly stating that they are impersonating actual celebrities. But since South Park is satire, and celebrities are famous, they can get away with doing so without too much legal trouble. (There have been cases where certain celebrities felt they stepped over the line and tried to sue but I don't think anyone has succeeded.) They could remove the disclaimer any time they want, and indeed not all episodes (or the movie) included it, and it wouldn't have much legal impact either way.