Gameplay-wise, I'll put it this way: Kirby games are about "using the enemies as your weapon" in two different ways. You can "grab" and "shoot" enemies, or you can "take their powers". Copy Kitty is all about taking their powers, while Psycutlery is all about grab and shoot. It makes the two games feel like they "rhyme", almost.
Copy Kitty's setting is kind of "superhero themed" as opposed to psionically themed, but there is still the story content of an older mentor-figure helping a younger person train/find the use of her powers. The personalities and storylines of the corresponding pairs of characters couldn't be much more different beyond that parallel, though.
I guess they also each have their own very particular visual style. Copy Kitty is exceedingly "lasers and explosions everywhere all the time sensory overload", to the point where watching gameplay can be hard to understand if you've never played the game for yourself.