Not that bad!
When I first saw the blank background on the title screen, it didn't look like it would be that good. While not particularly good, it's at least playable. The best part was how you had so much mobility and were able to easily shoot the enemies down. I think there might be something wrong with the audio because it seems out of sync. For a demo, this actually wasn't that bad. Demos can be good if you really put time and effort into them.
I suggest you not make the main ship so small. There could have also been more detail in the background with perhaps some stars. I don't know why Patrick and Spongebob appear there, but they're still cool. Their sprites are actually really well done, just to bring that up. You could maybe also make a wider variety of villians.