That's misleading. He only scored three two of which were against Ireland and one was a goal against an already spent Italy team.
Some other teams played against Ireland, too. Their strikers didn't go on to be top scorers.
That one goal was a goal in the final of one of the most, if not the most elite football championship in the world. And it was a striker's striker finish. So let's not downplay its value. And he could have scored another and be ranked first regardless of minutes played, but he decided to pass to Mata instead.
He's a striker, his job is to score goals. If there's one type of player that stats are more important for than others, it's the striker. He played about half of the total minutes his team played at that championship and he was still top scorer. I'd say that's not a bad job. Regardless of how "bad" he's really been.