This rings of my first post in this thread, abit a bit nicer.
Most media is average. Average doesn't equal poor. Average by its definition is the most available in this context.
Actual poor media does not sell well. Actual poor media goes quickly into obscurity.
There plenty of examples that are badly done media. fanfiction.net is an excellent resource of bad author. MST3k is an excellent resource of horrible movies. Plenty of horrible video games that Angry Video Game nerd has so kindly and colorfully remind us of its horribleness. Bad TV shows generally don't go onto its second season.
None of these examples sell well or will ever be sold due to its lack of quality.
Just because something sells well doesn't mean it's average quality. Plenty of badly made, creativly bankrupt things, with not a jot of worth, sell extremely well.
Now of course this is all just a matter of opinion. For every single piece of media created there will be someone who absolutely loves it, and someone who absolutely hates it. But just because theres someone out there who likes something, doesn't mean it's elevated from bad quality to average.
Look at the Star wars prequels for example. They sold extremely well, and yeah some people like them. But they are bad movies. Watch RedLetterMedia's reviews for reasons why, but as a quick summary: The first one doesnt have a protagonist, they are completely devoid of tension, the ending tries to rotate constantly between widly conflicting emotions over and over in a short space of time, and the plot makes absolutely no sense.
The scene he cut from the behind the scenes where Lucas first screens it for his producers and editors, and they all realise the ending is horribly made, but they cant fix it because it would require extensive reshoots is quite telling. It's a film that got made because George Lucas wanted to make it, and he's an influential director. It's a film that sold well based on the title of the film, the directors name, and the special effects. But the narrative, acting, pacing, and so many other aspects are just horrible. Not average, but just plain bad.
There are plenty other examples of poor media selling extremely well, this is just one of the easiest.