I'd say it's a mix. Fortunately, a lot of genuinely talented bands get picked up by persons in the music industry. But unfortunately, a lot of crap gets signed simply due to luck and who they know.
I think marketing has a good deal to do with it as well. If a record label thinks a particular artists is marketable (e.g. Avril Lavigne and her "punk" image etc) then they will give them a chance even if their music is terrible.
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