This implies they've been in (and left) or are attempting to be in fashion, though I prefer to believe they've escaped. (the difference being left-out or left-willingly, vs running away after being captured)
I've got a "Shrodinger's cat is dead" (with "not dead" on the back) shirt which helps me find interesting people (THE most random people understand it). I'd argue that's more useful than fashion (does it have a use?), and remember that stick-figure shirts occasionally explode in popularity (I remember a big burst several years ago). XKCD is just a superior-taste subset of those.
edit: heck, "fashion" hasn't served me too well anyway. I wore a blue-screen-of-death shirt to one of my interviews, and got the job. The interviewer called the boss in to laugh at the shirt :)
Fashion does have a use; the goal is to send social signals about status and group. While most people go way overboard and just follow along religiously, you too are in fact using fashion to socially signal like-minded people!
I wore an xkcd tie to prom with overwhelmingly positive response. Of course I go to a math/science magnet school (which really is just the high school where most of the smart people in the state go, regardless of their interests)
