I couldn't disagree more, I think it's a disservice, especially the way the music business is, to just disregard mixtapes as part of an artist's catalogue.
In the old days, mixtapes used to be the leftover tracks that didn't make the album. So instead of them going to waste, they'd just be put out along with a few freestyles over popular beats or whatever.
I know people hate to give him credit for anything, but Mood Muzik 2 kinda changed all that. After that, the whole "album was okay, I liked his mixtape better though" phenomenon seemed to get a lot more prevalent. Labels run a tighter ship these days and are trying to maximize every penny out of an album because of the decrease we've seen in sales, so a lot of quality shyt doesn't get used. Mixtapes have become an artist's own representation of their product rather than the album, which a label heavily dictates. Plus, you don't have to clear samples for mixtapes and can collab with whoever you want rather than being required to get label permission, so it just makes for a better product IMO.
With that being said, even taking Wayne's mixtape catalogue into account, he's just not a "top" rapper. Yea he was popular, but so was Luda, Nelly, Biz Markie, Vanilla Ice, etc. Obviously not for the same period of time or to that extent, but popularity doesn't equal legacy.
Dude just isn't as good of a rapper as the all time greats. As large as his discography is, how many concept songs does he have? How many crazy storytelling songs, etc? His substance is just not there to be held in the same regard as the Nas, Jay, etc. of the hip hop pantheon.