Maps come to Ranked because mappers are making maps with one goal in mind: to be ranked.
You can ask to any mapper (new or old) if want to see their last map ranked, and you will get a high % of "yes".
After many time mapping, they will perceive how difficult is to get a map ranked, so finally many mappers take a path: use the generic and sure style for ranking, or just don't try to rank their maps.
I agree with Xananda's comment. At this point of the game, we are thousands of mappers doing maps with the hope of get their maps in ranked, but it is usually a wasted of time because only like a 4~5% of set are ranked. But new mappers are not aware of this fact!
Following sergio's position ("ranking is a privilege, not a right"), then the ranking system must not go in the way of "anyone who make a map have the same rights, all persons are equals for the system", because this message is not true?
I mean, of course people are equal, It's for anyone that upload a map, but not for people that want to rank.
Here I think in two examples of "privileged people":
1) people that join to a Prestigious College or so, doing their best (or being a genius? xD).
2) people that know to the "correct person".
My point is, if people is worry to make their maps ranked, they will (and do) any thing to accomplish it!, of course they will get ranked maps, these generic and bored but high quality maps :3
But hey, it is not only about maps. It is about music too.
Some sets come to ranked not because the map or the mapper, but because the song is amazing, even if map is mediocre.
Are mediocre maps bad?
This question is... idk, I think that if the song is nice, not many players will complain about, but if the song is bad... xD
At end, high quality or mediocrity are stuffs that are not relevant if compare it with how fun is <insert a song here>.
If people played osu with bad maps in the past 10 years, thinking that high quality is the holy grail to save us of boredom, its just not true, imho xDD