Sales mean nothing. Pretty much anyone who played them will tell you Galaxy 2 is just as good or better than the first one. There are lots of reasons it might have sold worse than the first.
If sales mean nothing, go tell that to the folks in the sales thread. Sales mean a lot more than a review score. To suggest otherwise is to ignore the fact that games get made to be sold, not to generate nice words. Do you think Ubisoft really cares what the metacritic scores for the Just Dance games are?
SMG didn't generate interest in more SMG, that's why SMG2 didn't sell as well. It has nothing to do with review scores or which game people think is better.
Super Mario Galaxy 2 didn't sell well because it was released in 2010. There were a lot more gamers buying Wii games in 2007 than 2010. It probably didn't help that on the surface it looked like a rehash; you needed to play it to appreciate what new stuff it did. It was also released in the first half of a really stacked year, which meant it got less consideration in the busy holiday season, and only a few months after another Mario game, which most likely cannibalized its sales. As I said, there are a lot of reasons it might have sold less.
Regardless of that, sales don't matter at all in the context of this discussion, pertaining to what makes a game a classic. Pikmin 2 sold like ****, well worse than the first, but you'll find tons of people calling it a classic because the sales are irrelevant, what matters is what people who played it thought of it.
The Wii was still sold out at times in 2009, to say that the peak number of potential sales had passed by 2010 makes no sense--most people that bought a Wii did so after 2007. Everyone knows CoD games ARE rehashes and that doesn't stop 20+ million sales every year (with increasing sales every year), if SMG isn't outsold by 2, it's because people didn't want more SMG 1, rehash or not (and if it's not, well, name it something else). SMG2 and NSMBWii are 2 totally different games, to suggest that someone played a 2d mario game and that satisfied their desire to play a 3d mario game doesn't hold water. NO 3d mario game has ever sold like a 2d mario game, people clearly understand that they're 2 different genres.
Classics are games that remain relevant and even gain relevance as time goes on. It has nothing to do with being well regarded in the company of its contemporaries (although it might help find the game an audience)-- like most well rated and even good selling games. I'd hardly call the Band Camp/Guitar Hero games classic since the music genre has pretty much died out after a few years and I don't know anyone who still buys or even plays those games anymore. People raved about how good Okami was but it still helped put Clover out of business.
Sales matter because it give a game an initial base who the game might stay relevant to. The Pikmin games were never relevant. I'm not saying they're bad games but, unless some future Pikmin game becomes some smash hit, I don't see them springing back out of irrelevance. I mean, is Chibi Robo a classic? 1080 Avalanche? No, they were just okay games that most people didn't play when they were new, aren't playing now, and never will. It's not some terrible curse, they just aren't classics.