I'm probably the only one that is going to say that the music is one of the few things I disapprove of in the Mario series as of late.
The first games of the series, SMB1 - SMB3, have great soundtracks, no doubt about that. Each piece was original, jumpy, and great to listen to over and over again. Super Mario Lands 1 and 2 get mentions as well.
Super Mario World had a great main theme, but Kondo just remixed the piece over and over and over everywhere except the overworlds and the boss battles. That's my major negative about that game.
Then enters Super Mario 64, which has an astounding soundtrack, even if it comprises about 6 songs. Each piece had a sense of adventure to them, and made the game enjoyable.
But then, there's Super Mario Sunshine. God, if I could pick the Mario game with the worst soundtrack, this would be it. It felt so dry compared to SM64. The only redeemable songs from that game are Ricco Harbor, the boss theme, and the scat version of the original theme, imo.
The first two Paper Mario games contain the most stunning soundtracks I have ever heard. They are right up there with the Genesis Sonic and Sonic Unleashed soundtracks, to me. I could listen to Rogueport theme, Koopa Bros. Battle Theme, Shiver Mountain Theme, Doopliss Battle Theme, and a whole mess of others for HOURS. The songs really set the mood at the moment, gave the respective characters some personality, and are really great to listen to as you're fighting or running around.
The Super Paper Mario soundtrack, on the other hand, sucks. Not gonna sugarcoat that one. Not a single theme in that game is appealing to me, except possibly the Floro Sapien Caverns theme - not even the Mr. L theme or Super Dimentio Theme click for me. It's as if Kondo wasn't even trying to make good songs for this game, compared to the first two of the series.
The Mario & Luigi series had a kind of a on-again-off-again thing going on in their soundtracks. While some songs like BiS's final battle theme and SS's Popple/Fawful Theme are great, the overworld songs of SS and pretty much the entire PiT soundtrack except for the final theme are pretty sub-par.
The New Super Mario Bros. series falls under the same thing Super Mario World did - great main theme, but remixing the **** thing repeatedly for almost every stage is a major downer. NSMBWii get's a plus for remixing songs from previous games though, albeit it takes the Airship theme 30 seconds to get to the main part.
Since Motoi Sakuraba was in charge of the sports soundtracks, one can expect them to be good. And good they are. No argument there.
Mario Kart series has some of the most memorable songs, but then, it has some lackluster songs as well. It has all the Rainbow Road pieces, Sky Garden of Super Circuit. and Luigi Circuit from MK64, but then I can hardly remember much from the other themes for the other tracks.
Super Mario Galaxy. Orchestra makes everything better. No real arguments over this soundtrack, except perhaps they could have been more pronounced and adventurous in some of the songs, like that Bee Galaxy song.
tl;dr: The Mario soundtracks seems to have an on-and-off sense of quality to me - the quality doesn't remain consistent for the most part, unlike the Sonic and Kirby soundtracks. At times, like the sports titles, the Paper Marios, SM64, and the 8-bit games, the music is spectacular. But then with Super Paper Mario, Super Mario Sunshine, and the NSMB series, it feels very lackluster and not all that great.