The game felt pretty smooth and the levels with the UFO were pretty fun, however, I suggest you not to use spites with different scales, if you make a pixel art game, everything has to be pixel art, also, in the last level, the background looked pretty weird, I suggest you to just take an image and making it follow the camera, and I am pretty sure you could use a parallax background ( there are some tutorials on youtube on how to do it ), but for being your first game, I think it's pretty good, good job!