If you photograph or illustrate a celeb and they're in the public domain, I don't see why you couldn't use that on a shirt design.
"Public domain" does not refer to public figures, it refers to works
that are not covered by intellectual property rights.
While copyrights or trademarks could be released into public domain for a variety of reasons, it is very rare for a person to release their own name and likeness (keep in mind, right of publicity is not intellectual property gained by creating or selling something or registering with a federal authority, it is legal right given to all people). In fact, it is more common for a celebrity to increase
the value of their image by licensing it, not decreasing
the value by releasing it to public domain.
For images that are in the public domain, it just means that the copyright of the image has been released so that others are allowed to use it for any purpose. But that does not
grant any license to use any intellectual property that is within the content of the image. For instance, if I take a pic of Derek Jeter and release it into the public domain, that does not mean someone can make Derek Jeter t-shirts. His right of publicity still applies and the legalities of Major League Baseball trademarks, copyrights and merchandise licenses still apply.
Politicians are fair game, why not other famous people?
Politicians are not
fair game. They still have right of publicity just like anyone else. They just choose
to not pursue legal action when their rights are violated. But they in no way officially waive their legal rights.