I don't think Cain got a bad chin(like Arlovski), but usually (long-term) champions are guys with iron chins or else they end up losing because of a "fluke". I think Cain's chin is average, but not great for a heavyweight.
For example, if JDS makes a mistake and gets clipped he will probably absorb it, while cain will probably get woobly and possibly koed.
I think the only dominant champion without an iron chin is GSP, and he lost against Serra because of that. Guys like Junior, BJ Penn and Anderson Silva can make mistakes once in a while.