I think, but I’m not sure, that the “xx/120” counter is in fact number of chests found, not number of levels completed. You see, I went back and unlocked another secret level, and completed that, and the counter went up from 76 to 81.
I still have a ? for part of the collection screen too. I assume I get something there for getting all of the treasure chests or something. Or maybe it’s to do with that yellow Kirby I see sometimes but never manage to catch.