mandu, with respect to your opinion, to me Jack Johnson is a lightweight whose music is forgettable. He will leave no indelible mark on the music industry.
He has made some fine songs. I'm not that into him, but the things I've heard makes me feel 'good', which undoubtly is the whole point of his music. It sounds like the music you're playing when you've smoked joints and feels to chill out.
I'll doubt he will go over in music history, but less can do it. It's fine music.