That seems like a trivial thing to base the decision on to me.
I suppose you've got a pretty casual work environment, including the dress code, right? And you're worried about how formal they've dressed and that it's in stark contrast to the culture you're building?
I suppose it could be a small hint, but I honestly wouldn't stress over it too much. The person probably didn't know what dress would be most appropriate (it might have been better had they asked, but they didn't) and decided it would be better to be over-dressed rather than under-dressed. (That part is probably true.)
You have to keep in mind that there are some parts of the country where if you work a desk job, you wear a suit. That may be where the candidate is from. Or he/she may be coming from a company where that is what's expected, and not know any different.
There's a much broader picture to be painted for cultural fit. Don't stress out if they overdressed for the interview.