What are you? 16? Besides, their Debut sold 3 freaking million copies, dude. Id say thats pretty mainstream, right outta the gate.
I don't like speaking for someone else, but I believe the context of mainstream being used here is how their sound had become more commercial than their first 2 albums.
Songs like "Dope Nose", "Hash Pipe", "Beverly Hills", and "Island In the Sun" were inferior to their earlier work with the band intending to compromise themselves for commercial gain.. whereas the earlier material was a lot more creative and contained a lot of substance...