I think it's a good idea. I expect David Cameron is intelligent enough to know that this won't win him votes, especially as the people it is aimed at are below voting age.
I have criticised the Conservatives a few times on twitter even though I mainly support them, so I'm not one of those who would just say it's a good idea even when I might think it's not. It's worth noting that the contest will be run by people from the music industry and not by politicians.
It's about promoting young talent. Somebody has to be lined up to replace the bitter old sods who's run has long since come to an end.
(excolet on twitter)