He did, but Obama attacked Romney's personal character and integrity far more than Romney attacked Obama's. Romney focused on the record.
Maybe, if you say so. On the other hand, Romney made much more false/misleading claims, I think most objective fact-checkers agreed on that. Obama's attacks on Romney were mostly unveiling parts of his character which weren't yet known by the voter. And the acts most damaging to Romney's image were made by Romney himself (47% anyone?).
Point is, campaigns aren't clean and this one was nastier than usual. But a Romneyite whining that Obama won by being nasty is plain ludicrous.