I don't know which is worse, honestly. We expected NSMB2 to be mundane from day 1. And while it's arguably the weakest Mario platformer to date, it's hardly a disappointment when no one had any expectations to begin with. Whereas Sticker Star had tons of hype and basically everyone on GameFAQs was blabbing on about how Sticker Star would be the REAL Mario handheld must-buy this year. And so far, the reception has been way less than stellar.
Luckily NSMBU is so far cleansing the sour aftertaste of Mario's uncharacteristic handheld sins this year (I guess that includes Tennis Open too).
If you look at this objectively and not with rose tinted goggles, you clearly see that NSMB2 was generally not that great of a game. It was good, but just good.
PMSS on the other hand is a great game, again if you look at it without preconceptions of what YOU think it SHOULD be. This game has gorgeous art, top notch music and is just generally fun. NSMB2 felt like a bunch of left over NSB levels and content stitched together for money.
Here's the thing about NSMB2. I think the soundtrack is one of the most disappointing because outside of a couple of remixes of remixes, the music was a straight up port from NSMB Wii. That said, the levels were some of the best to me out of all three games when playing solo. I mean, it even managed to make auto-scrolling stages fun. I had a blast with it and loved how they used the design to throw in hidden blocks. You just knew there would be a secret there kind of stuff.
I'd probably give NSMB2 a B+ whereas I'd give Sticker Star to this point either a B or a B- because it does a poor job of preparing you for what's to come. You might not even have the right sticker setup if you're not careful, but the levels themselves are also fun and pretty varied. It also has a much better soundtrack, but I think that goes without saying.