The picture works for me quite nicely as-is. Whether the apparent horizon is tilted or no, by mistake or intentionally, is not that important for me. The impression I get (and like) is of a gently sloping hillside. It also clear (to me, of course) that the image is not meant to be documentary, but rather an interpretation, and as such is allowed to use artistic license, including tilting the horizon (if that was intentional).
And for the record, I use quite often a bubble-level for my own landscapes, when it would be glaringly clear that tilted horizon is a result of me being sloppy, not terrain sloping Now that I know there is a bubble-level police out there, I will be twice as careful.