While mild compared to most of the adult doujinshi I've seen men reading on the Tokyo subways, Welcome to the NHK! does have very suggestive and graphic scenes -- just about all of which are happening in the main (male) character's imagination. There is nudity, as well as scenes that would be too explicit for, say, an R-rated film. Some exchanges (mostly in the discussions of the H-game in development) are demeaning and denigrating of women -- perhaps true to context, but nonetheless not uplifting, pure and noble.
The anime was very, very toned down from the manga. I found the anime to be a little edgy, but the manga goes far beyond that.
In the context of Japanese teen culture, perhaps this is mainstream, but it definitely is beyond anything you should expect to find in American publications.
Does that answer your question?
"If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world." -- C.S. Lewis