The whole "too serious, too blah blah blah" thing was the inevitable backlash from overexposure -- all the acclaim, all the success, all the fawning media attention that came after the release of JT.
I can't recall what I was just reading ("the bible," I think), and Bono said he was getting those "red flags" during the ZooTV/Zooropa tours that told him they were veering toward overexposure. He mentioned those red flags came up around the time of "Rattle and Hum," but ignored them.
And I totally laughed at that interview where the Edge half-jokingly said he would like to apologize for R&H. Hee!
I was gaga for U2 when R&H came out, and I saw it several times in the theaters. I still watch the DVD and enjoy it, but lately I just burst into giggles when I watch their serious, dour expressions. Yeah, there's bits of humor in it, but it just cracks me up.
But then, by the last half hour or so of the movie, it's all music, and the giggles and eye-rolling have gone away, and it's just wonderful. And that, after all, is the point.