A friend of mine is an engineering consultant (software) and looks shabby. He met me for lunch one day after seeing a client and I said "you went to see them like that?" He said it's part of the uniform. It sends a message about being good enough not to worry about superficial impressions. I'm sure in some cases that's true.
That got really hammered home when I showed up to a job interview in a button-down shirt and slacks and I was _far_ more dressed-up than the CEO. Getting bounced for not being a "culture fit" cleared up any lingering ambiguity.
I was pleasantly surprised when I completed my talk and the guy who heckled me (I never even saw him) came up to me and apologized. Then again this was in Toronto so maybe the old stereotype about apologetic Canadians is more true than not!
The "shabby nerd" stereotype is a very real thing and it goes both ways.
I always reply with "every day is a job interview, baby" and do cheesy finger guns. That shuts them up.