A substantial minority prefers `outdoorsy' clothing --- hiking boots ("in case a mountain should suddenly spring up in the machine room", as one famous parody put it), khakis, lumberjack or chamois shirts, and the like.
Very few actually fit the "National Lampoon" Nerd stereotype, though it lingers on at MIT and may have been more common before 1975. These days, backpacks are more common than briefcases, and the hacker `look' is more whole-earth than whole-polyester.
Hackers dress for comfort, function, and minimal maintenance hassles rather than for appearance (some, perhaps unfortunately, take this to extremes and neglect personal hygiene). They have a very low tolerance of suits and other `business' attire; in fact, it is not uncommon for hackers to quit a job rather than conform to a dress code.
Female hackers almost never wear visible makeup, and many use none at all.