I haven't heard that track in particular, but I just say:
1) I probably wouldn't call Snoop a bad rapper- within most circles, he's considered to be one of the elite. That said, even the best performer can toss out a clunker. I'm sure JM will have some words with Snoop in the afterlife.
2) Many rappers have been doing just this for about a decade...after all, remember what Puffy did with the Police hit, "Every Breath You Take" when he turned it into a (bad) rap-eulogy for his fallen buddy Biggie? He even had Sting reprise his vocals on it during a live performance. It all really started with Run DMC's partnership with Aerosmith on a remix of "Walk this Way," soon followed by Anthrax & Public Enemy redoing PE's song, "Bring tha Noize" and the landmark "Judgement Night" soundtrack.
That said, it doesn't make them any better. IMHO, I've heard only a few rap/rock remixes that I find even tolerable. Most rap/rock fusions are best when they're entirely original.
3) Snoop probably didn't mean it as anything but a tribute. He's gone on record as being a big fan of a LOT of music you wouldn't normally associate with a giant in the gangsta rap genre- he was recently on Larry King Live commenting on his love of Johnny Cash, for instance.