Well, as a Japanese game, Japan is ALL ABOUT crossovers. Square Enix, even moreso.

If I see any of this stuff in game, I just chalk it up to whimsy. I mean we have the Slime King crown, an exaggerated, phallic pompadour hairstyle, a flying broomstick, legendary beasts as common mounts, and a pig mascot costume. Sure, maybe they don't quite fit into the game's world, but we have plenty of weird things that are part of the lore and world; Captain Longhaft, anything remotely related to Hildebrand, an entire noble house dedicated to playing match-maker, Leviabeetus, a goblin cult that has sacred cheese, and a wandering pompadour hitman squad... so when you look at it from that perspective, relatively, cartoony monster minions and weapons aren't so out of place.