I like the art and the quirkiness of the cat in general, but boy am I terrible at those games XD Managed to clear it on Beginner difficulty and nothing else.
I am, however, impressed you implemented several difficulty levels.
For me, the interface, while pretty in itself, was visually at odds with the idea of eldritch piano music. I think I would've preferred something a little closer to diegetic interface elements. Additionally, the response on hitting a note was basically non-existent. Some sparkle or at least a much brighter highlight would help, anything to indicated the player did it correctly, given there really isn't much room to experiment.