No one's stopping you from using it, in fact I think it looks very nice, however I'm not sure if it fits the definition of pixel art that's commonly agreed to here - that the individual pixel is considered when making the art. To a degree it fits it, and to a degree it doesn't, but I'm going to have to say this is more of a digital painting than anything.
Edit: Now that I have more time to examine it, I'm going to add to what I said before. Color conservation isn't actually essential to pixel art, and from what I'm seeing it looks like after you sketch (as in the clouds up near the top) you go in with a single pixel brush, which again isn't necessary for being pixel art, but helps the artist concentrate on the pixel, this does actually meet many of the criteria, however loose, to be pixel art. In my completely honest opinion. It could definitely benefit from refinement, especially a more opaque one.
I think the 20% opacity is okay actually, because you know whats going to happen because there's no auto AA or anything.