Grunge was a style of dress which was very popular for a couple of years back in the early 1990s, and had at least one thing in common with most of the fashionable styles throughout the ages. Since it dealt with the “slacker” class of people (the scions of middle and upper class parents), they were not officially poor, even though they practiced the values most characteristic of poverty. For the people who lived the grunge life style, it was all about living in the moment and not remembering very much of what you did last night the next day. Simply put, grunge was a celebration of college and early 20-something (generally unemployed) partying slacker culture, and the entire name of the style basically says it all. When you wore grunge, you were permitted to do a lot of things with your personal hygeine which would normally never fly in fashionable circles.
Much of society works to venerate people who are essentially at the bottom by their own choice. The type of slacker who made grunge into what it was did so because they wanted to be drunk a lot of the time, and consequently rarely had the time to do such bothersome things as bathing, brushing their teeth and actually changing their clothes. In fact, it was almost considered a badge of honor to have old beer and snack stains on your grungy outfit, provided that you had done everything that you could to get the alcohol out- because wasting booze was tantamount to sin.
Grunge almost carried a redneck vibe to it, with its messy and ill kempt hair and heavy reliance on flannel as both an accessory of fashion and as a hedge against occasionally falling asleep (read passing out) in outdoor spaces. If you were going to party in the early 90s, grunge was a very safe bet, both for the functional reasons of dressing in clothes you would not mind throwing up on (as well as having other people throw up on) and simply to look the part of a proper, parents-have-money child of partying.