The multiplayer reminds me of what Valve said when they were asked why they chose the current art style for Team Fortress 2. They said multiplayer gaming, no matter how serious the context, is inheritably funny. Funny, unbelievable shit happens due to the game environment and everybody laughs. They chose that art style to encourage a more relaxed, humorous tone to the game world.
NSMB Wii is pretty much exactly the same. In reality, the multiplayer will have you consistently dieing, screwing up, and deliberately hindering other players that are ultimately trying to accomplish the exact same goal. In a single player scenario dieing becomes frustrating, whereas in multiplayer its hilarious.
It's like a slapstick duo, the characters like Ren & Stimpy constantly doing each other over despite being a 'team'. You can lose tons of lives on a single level and laugh the whole time.
That's what makes it so great.