I just saw this exhibit at our MoMA, and the work is outstanding. Some of the images are color, some black and white. There are a number of styles and artists, ranging from the very famous to those unknown outside of their small communities. The point is that these are images of musicians--it's that simple.
Sometimes, Liebovitz's work is witty, sometimes it is unflinching in its honest portrayal. There is vulnerability in the subjects of her black and whites because they are so close, often just the face of the subject. To term them "ugly" is simply wrong. It is rare to see behind the artifice of celebrity images and see performers without makeup and with their skin texture and pores visible. Some of the photos are taken in people's homes, or backstage rather than on a set. This lends considerably to the intimacy and honesty that she is trying to convey.
If you want shots of your favorite singer looking oh so pretty, go to their PR person. This is a serious body of work from a renowned photographer. It blends both her celebrity work with her own private interests in portrait photography for non-commercial audiences.