I went back and played a bit again to remind myself of what gave me mixed feelings about the art. I played fullscreen the first time... this time I went back and forth a bit, and I do think it works better on the small screen... though I prefer playing in fullscreen. Fullscreen really causes the BG images to break down unfortunately.
Beyond that, I think the BG art itself looks solid and has a distinct style going for it. However, it doesn't quite match the cleaner character art and in-level clean sprites. Since I posted my comment, XCVG played and commented, and I think they nailed the problem on the head. The art is all good individually, but there's a big mish-mash of styles that make it seem a little off.