You raise an interesting point here - why is Kenny G's clarinet so insufferably cheesy to so many (including me), but Danger Mouse doing the same thing with a drum machine is hailed as a visionary (though I'm personally indifferent).
I think the difference may be that while DM is no different, his contributions are at least contemporary; that doesn't make it better, just more fashionable. Clarinets used to be fashionable, and Kenny g's stock-in-trade is musical nostalgia: so nostalgia^2 is not an improvement to most people.
On the other hand, this http://www.youtube.com/watch?v=EJ9iNoTKMAs (same song mashed up with Radiohead) presents the same melody in a different but interesting harmonic context - that is, in the same key but with different chords behind it. To me that's innovative, although I don't personally like Radiohead. On the other hand Kenny G's clarinet vampings seem completely unoriginal - as someone who uses sequencers because I'm poor at actually playing bass or piano, he's much better than me at operating his instrument, but isn't doing anything remotely novel with that ability (like introducing new rhythmic or harmonic tension, he's just echoing what's already there with a clarinet).
Although we disagree, I think your points are interesting because I find myself wondering about things like this when I reach a site that mashes up (say) Craigslist with Google - is there really anything original going on here, or is it just digital glue, something the browser will do by itself in a few years?