I would most likely mention it in the interview. However I would mention as a sidenote or on my way out at the end of the interview. I'd probably say something along the lines of "I would have written the code slightly differently" and explain why. It is possible that the code was written by a current employee (or an interviewer) and not neccessarilly written badly on purpose so this could in a way discredit them, but I don't think the famous Douglas Adams saying "noone likes a smartass" applies here. At least I wouldn't want to work in the environment where that saying was true.