If I had to guess, there are both positive and negative effects on celebrities as scientists, dependent upon enough factors that there's no good way to make a headline. The effects a celebrity scientist has are dependent upon why people identify with them, how the public reacts, and of course what the scientist does. If the results of celebrity scientists are making cool posters for dorm rooms and/or being eye candy, then yeah, they probably aren't doing much for it. But, if they are testifying before Congress to act on scientific data or fund research, or encouraging people to improve their critical thinking skills, they are immensely helpful. It's also important that they stay on that side of the line. Discovery Channel and shows on the Discovery Channel have had issues with that.
If you really want to advance scientific literacy, you're going to have to dispel the idea that it's common for something to have virtually only positives or only negatives, as in reality, those kinds of things are quite rare.