Partisanship is such a driving force in our politics that even someone who only bucks their party 4% of the time is considered "bipartisan". I've seen reports in the media calling a vote "bipartisan" if it only gets two or three votes from the other side.

And yes, it depends on that person's reputation. Since Collins is seen as a moderate, all it takes is her support on a bill to call it "bipartisan". OTOH, Senator Rand Paul frequently votes against the Republican party line, but only because he is a batshit crazy libertarian. He can vote against a Republican bill, and nobody calls that "bipartisan", I guess because both sides think he's nuts.