Broken = unstoppable, unbeatable, or so difficult to overcome that one side uses a disproportionate amount of effort to the other during a match. Tournament results and testing indicate that the infinite is not, in fact, broken according to that definition.
...Enough of that. I think they would need to clean up Nana's AI so she doesn't, without warning, attain super timing on her aerials, dash attacks, dodges, etc. Whether she's a really powerful AI or not isn't really the issue; the problem is she becomes impossible to predict because there are few patterns to her behavior. Sometimes she goes running straight through an opponent to return to Popo; sometimes she just attacks them blindly. Sometimes she stays on the other side of the level totally inert, then powershields and rolls and blizzards her way to safety when she's attacked. I don't care what the level of AI is, just please be consistent.