I would say that although the media does focus on a lot of negative elements of our society, if it were not for the media, it would be more difficult for us to find out what is going on in society.
Whereas beauty magazines often create unrealistic portrayals of women's body types, the news media call our attention to negative portrayals of women (e.g., Herman Cain's "Princess Nancy" comment).
While the media does spend an unusual amount of time showing us unimportant things like what celebrities wear, the media also pays attention to what celebrities say and do, which can bring attention to important issues. Because actor Michael J. Fox has Parkinson's Disease, this has drawn the media's attention to this disease and more work is being done to help remedy this disease that might otherwise not have occurred if a celebrity had not brought attention to it.
Likewise, because the media frequently shows violence, this can draw attention to the problem and hopefully prompt people to do something to remedy the problem. The media makes us aware of violence and wrongdoing that we otherwise would never have had any knowledge of. The problem of bullying in schools has become a topic of national conversation and action thanks, in at least part, to the media.
Finally, with respect to stereotypes, I think the media has done a great deal to soften prejudices against stereotypes. Television media has made a strong effort to make its newscasters reflect the diverse elements of society, especially with respect to race. Even television programs have helped change attitudes toward people who are stereotyped by at least putting such groups before the eyes of the common viewer and letting the public at large observe groups that they otherwise may not have ever seen.