Kirby thought it was King Dedede had stolen his cake because he thought he saw a waddle dee, King Dedede's servant, take it.
Meta Knight knew how dangerous Dark Nebula was, but wanted to give Kirby more of a test than the one in adventure, but it got weakened after many years sealed away. Plus, if you count Dark Daroach and Dark Nebula as the final boss then it's the same length as other final bosses. Overall Mouse Attack was meh and one of the most mediocre Kirby games.