Nirvana may have been trying to make a statement that they were different with there style. I have seen this with many of the bands that I have listened and seen in concert seem to have a drastic style to get attention. Some times the members in the bands will wear a ton of black makeup and paint, with black hair styled crazily. Other times they will have many different colors in their hair. Some have tattoos covering their bodies and multiple piercings. A lot of them wear dark clothing with chains and things on their outfits. These bands are all possibly trying to say "hey check us out we are different" with their styles. Though many of these bands tend to have styles that start to overlap with slight changes so they do not stand out as much. But these bands that are copying others may be trying to make a statement that their music is similar to the band that started that style, and if you like that band you should listen to them as well. Which may be true because the bands that look similar tend to be on tours together, or have music that has something in common with the other bands. It may be that it is not a statement that the bands are similar at all, but due to the fact that a member from another band joined them and they adopted that image. All bands have an image and they all want to catch peoples attention with it. They make a statement with style rather than words.
Maybe people should start looking at the music they listen to and the artists that make it. Then find the music that is similar and compare the artists and see if they have similar styles. If you find one will similar music but a way different style you should look more closely into them and try to figure out what stamens they are trying to make.