You might wonder what the Dimension Mirror is doing in the final boss's room. Kirby and Meta Knight had to fly up into the sky to get to the mirror, which certainly put it into Sectonia's domain and reach, and because Queen Sectonia is so vain, she has probably been using it as a personal looking glass. It may not be why she's so evil, but it could be one explanation for how she became so monstrously vain.
The developer's talk in Miiverse basically confirms this, with additions: It was Taranza who gave it to Sectonia as a gift, back when she was a good guy; the thing just happened to contain a vengeful Dark Meta Knight, which corrupted Sectonia and turned her into the evil, vain queen she is now.
King Dedede being the Hero of Another Story in this game and getting such a huge presence is actually one gigantic pun. Think about it: Kirby Triple Deluxe. As in, Kirby 3D. Or rather, Kirby DDD.
Why does Amazing Mirror get such a focus at the end of Dedede Tour? It's the tenth anniversary of Amazing Mirror AND it makes up for Dedede's absence in that game!
Why does Taranza just assume that King Dedede is the Hero of Dream Land without anything else to go off of? Just take one look at Sectonia. She's immensely powerful and, as we know, was once a just ruler before the Dimension Mirror more or less broke her sanity. Taranza assumes Dream Land's ruler to be its hero because their ruler was their hero.
Even though Taranza was wrong objectively, he ultimately ends up having made the correct choice in heroes, as both Dedede and Kirby save the People of the Sky from their corrupted queen.
Can double as Fridge Horror: Queen Sectonia is referred to and designed after a species of parasitic wasps — in the game, they are said to use their parasitic nature to subjugate others. Some of these wasps are known to specifically target spiders. Taranza is a spider/tarantula. Ever wondered why Taranza was so easily under her thumb?
This becomes worse with The Reveal in Planet Robobot that Sectonia's true form is that of a spider, especially when you remember Dark Meta Knight's nature as The Corrupter in Triple Deluxe, because Dark Meta Knight likely picked up the wasp form and convinced Sectonia it's beautiful specifically to keep Sectonia under his thumb, and Sectonia, whose delusions of grandeur turned her into a Horrible Judge of Character, turned herself into Dark Meta Knight's puppet without realizing it.
You'd think Taranza was safe before Sectonia's corruption, right? Wrong! Female spiders eat their mates. This is actually why Taranza was so easily under Sectonia's thumb. Of course, it's not impossible to apply Artistic License – Biology; in fact, most shippers do.
Queen Sectonia's appearance is based on a wasp, namely a queen wasp. Normally, queen wasps live up to only twelve months, which basically equals a year. Can also double as Fridge Horror and Tear Jerker after you learn about Queen Sectonia's goal to become a goddess as well as her corruption into a tyrant through obsessive gazing into the Dimension Mirror.
Later in the game, we're told that Kirby was chosen specifically by the Dreamstalk to save Floralia. So why does both Kirby's house and Dedede's castle get lifted? Well, either a) the Dreamstalk knew Kirby needed motivation to go into Floralia, or b) both Kirby and Dedede would be fit for the task. Keep in mind, this game takes place relatively soon after Kirby and Dedede saved Popstar/the universe, and Dededetour shows that the King, as long as he has his hammer, is fully capable of saving Floralia by himself.
Why is the Dimension Mirror the force behind Sectonia? Because this is the first Kirby game where you travel between the background and foreground, or in the third Dimension!
The games' lore has implied that no matter how many times Kracko is destroyed, he can always revive on his own as long as there are clouds in the sky. So, why does Taranza have to cast a spell to revive him through a cloud? Because, in the last game the real Kracko appeared in, Kirby & The Amazing Mirror, there were no clouds in his battle arena, in a first for the series. The three Kracko battles in-between were a clone painted by Drawcia, a mech piloted by Doc, and a yarn monster created by Yin-Yarn.
Up until this game, the only connection between Dark Meta Knight and Dark Matter was the former's subordination to Dark Mind, who many believe to be a Dark Matter. But take a close listen to "Sullied Grace" (Sectonia Phase 1), and you can hear snippets of 02's theme. With Dark Mind dead, this implies that Dark Meta Knight himself may be a Dark Matter... Kirby Star Allies supports this possibility; after all, many people believe Kirby and Meta Knight to be the same species, and they have the same progenitor as Dark Matter.
