We must understand if he can't perform as well as he performed in Bayern Munich previously, he is getting old and he played with a different tactic & with different players' characteristics. So, we can't expect he should perform as he ever did in Bayern Munich.
By the way, he may miss some easy chances. But it doesn't mean he played badly the whole season, he was probably unlucky to execute it. Don't judge his performance based on a few matches only!! Being the top scorer in La Liga, is proof that he still played well (as expected).
Thus we have to look at his overall performance with Barcelona this season, indeed he failed to provide anything positive for Barcelona in the ucl and copa del rey but he was able to make a big contribution in laliga which is perfect enough in my opinion.
Barcelona will immediately lock the title in the upcoming match when they face Espanyol, Espanyol is not a strong team this season, I think barcelona can easily beat them and lock the title in this match.