Well, CoM wasn't actually a spin-off. It's important to the main plot between 1 and 2, because in II, they never explain why Sora just wakes up in that white room.
You still need to play it to understand the story. All the games are canon, so none are technically spin offs. Birth by Sleep is very important as well.
Yes, it was originally a mobile game, now an NDS game. And it is canon. IT relates to Mickey's letter, what was on the letter, and how Yen Sid wanted to bring Sora and Riku into the Mark of Mastery exam, which was the teaser in the NDS version. So yes, it's important to the plot.