Politicians are no better than most rock stars. Plus it depends on the actual person and not just a general grouping of rock stars and politicians. Some politicians may be better than others at being a role model, just like some rock stars try hard to create a positive influence.
There are some musicians -- even rock stars -- who might be considered role models. Just as there are some politicians -- admittedly, even fewer than there are rock stars -- who could be considered role models. It's important to judge individuals for the contributions they make to better our society, and not a profession as a whole. Generalizations have never helped anyone.