Unlocking fighting game characters in this day and age is, in my opinion, obsolete.
You're gonna wanna try out the entire cast or at least that one character you like. It wouldn't be fair if your potential main is locked away or, worse, DLC. Things like having to clear something with a certain character to get other characters (i.e. Young Link in Melee) is a really bad incentive to try the rest of the cast. It makes sense to a degree, but if you're someone that generally knows what they're looking for in a character, it's no fun at all.
I'm all for unlockable music,trophies, stickers, and maybe even trophies. I think, however, that the core of the game, the characters, need to be available immediately. Now should they have made Rosalina unlockable in 3D World? I don't really think so, but there are more differences in a fighting game cast than there are between the 3D World roster so the comparison is not QUITE the same.