Believe it or not, celebrity testimony before Congress in most cases is a very good thing. For starters, most celebrities who do appear before congress are actually bona fide experts and passionate activists for the causes they care about. It also doesn't hurt to have a famous face attached to something to raise awareness. In a best-case scenario, a popular celebrity can draw mass attention to an issue and actually create some real legal action in Congress. In some cases, celebs are actually invited by Congress to speak on behalf of a current issue that's being addressed.
Here's a list of celebrities who have testified before Congress. You might be surprised at some of the names here.