Nice idea
I'd say as an experiment, you did quite well.
On my first play through, I died a couple of times trying to understand what I was doing, but I figured it out -- not that it's hard. I liked fighting the giant hourglass in level 2; I hate the hourglass, always tellin' me to wait.
I found the game to be an enjoyable distraction. The music went along with what was going on, and that's always helpful. The powerups were pretty neat, especially the wall-drawing pencil. So I am lead to believe that this "experiment" could most certainly be expanded upon. There is a varied amount of enemies, interesting powerups, familiar (and hated) bosses (I'm lookin' at you, Hourglass) -- there's a formula for success, here. Like it has been said, it needs more substance. It could stand to be a bit more fast-paced, as well.
Under this idea, there could be a lot of cool additions, such as icons, buttons, pop-up windows that you can close, resize, move.... I don't know.... there are plenty of things the ol' mouse cursor can interact with, and turning it into a shmup is cool.
Lots of potential here. My score reflects that this work is an acknowledged "experiment" that turned out to be game worth playing a few times. Cool.