As long as he keeps up with the music and touring (far more important than some reality show), I have no issues with it. That having been said, I think it'd be a bad idea. American Idol does not have much credibility left (not that it ever really had any to begin with) and I can only see Bono joining AI hurting U2's reputation as a band. However, if Bono were to be a mentor or guest judge, that would be a far better option.
Same can be said if Jagger joins AI as a judge. Would only do damage to the RS reputation, imo.