Yes, Steam Greenlight has quality control issues, but it still has a purpose of getting games to the store without going through a publisher.
The environment has been better-than-ever for indies, especially as increasing costs make AAA games harder than ever to make. (in 2015 alone, we have Rocket League, Ark: Survival Evolved, and Undertale)
I think if a developer is relying only on Steam Greenlight to market their game then they'll likely have issues, but if they market in other ways then (if their game is worthwhile) they'll quickly build an audience.
Rather than drowning out good games, it is becoming easier to tell which games have some love put into them. It reminds me of the movie Idiocracy[0]. What might have been a C game before Greenlight suddenly looks pretty damn good compared to all the F's floating around them.