For a game I'm assuming was developed in a short space of time, it's a fun little game but it does get tedious quite quickly. I don't know if it's just my laptop, but the animation was a bit jumpy and put me off too.
I do agree that making use of the UP key, as well as adding in perfect models, although I wouldn't know how you would get penalised for that considering you don't for hitting a key when no catgirl is in front.
The underlying problem of the ease is that at maximum speed, it becomes more of a keep-the-rhythm. I can pretty much start to stare into the screen and keep up okay. Something to keep the player on their toes would definitely make a difference to that.