I had fun playing it. For its basic design it's good. From here you can look to upgrade it.
I ended up mapping the keys to my game controller to get a feel for it. When i got around 700s or slightly before, the mobs started becoming a bit too hard to hit. I tired my arm trying to press the shoot button as fast as I could, and just couldn't keep up. This is where the power ups idea would be great... or maybe a hold and release shot or something.
I like the ideas amannamedjayne mentioned. Namely, the powerups and bosses would help. A simple story such as, "Nyan cat was just nyanning around when her flight was interupted by evil Nayn cats!" or something silly like that could suffice really.
Overall, I liked this as a simple way to try and last through the song as long as you can while having something to do (even though you can mute it, ha!)