I understand that interviewing many people is probably a frustrating process, but I still think such a comment is out of line. I am sure at least some of the people who do not dress up do not know any better. None of the professors that I knew while a graduate student had ever attended the joint meetings, let alone interviewed for a small school position. If you don't have advice from colleagues and don't have friends that have been through the process, I can understand making the mistake. Every job candidate that came through the school I was attending while a graduate student wore jeans and a t-shirt or such to their interview and that was fine.

I interviewed with several schools (research and teaching) where the people doing the interviewing were not dressed up. Given this I definitely don't think there is a set dress code for such interviews.

I also think it is important to keep in mind the process isn't pleasant for the person being interviewed either. I recall several schools telling me one thing on the phone and finding out it was not like that at all once I wasted the time (and on one occasion airfare for my wife) to visit the university.

I'm at a public liberal arts school with seven mathematicians. I won't speak for my colleagues, but I've never had an issue with any candidate's dress. When we host someone for an on campus interview, we tend to dress a little nicer than usual (I put the running shoes away for a day) if only to make the candidate feel comfortable.