It is always more entertaining to play a game revolving around some adventure (as is the case of Mario) when it has a compelling story to boot, no matter how kiddy and simplicistic.
But it shouldn't be boring, which it could be by being too repetitive ("been there, done that") or too long in exposition (some people complain that the admittedly rather forced plot expositions in Galaxy were a chore. I didn't feel like that, but I can see why they would).