Cristiano Ronaldo actually scored many goals for Al-Nassr but the club did not win any official title, and it's not only responsibility of Cristiano Ronaldo. His team mates are responsible for failure of the club in last two seasons.
Yes, if you look at other teams now, their performance is indeed better than Al Nassr, such as Al Hilal and Ittihad. But yes, we'll see whether Pioli's troops will rise or not.
Al Nassr was one of the best teams in Saudi Arabia even without Ronaldo before he joined this team but they hired Ronldo and they were hoping to see much better performance from this team even though they were hoping to win the Saudi League title with Ronaldo in his team.
But the fact about Ronaldo, when they hired this player, is Al Nassr didn't just hire Ronaldo to have better performance in this team but Ronaldo is much more than just a player in this team.