It's an old debate. What exactly is nature photography? Try googling that question; you'll get hundreds of attempts at definitions. As elected leader of Biofoto, a Norwegian Society for Nature Photography, I've been forced to think this over a lot of times, and to try to find a tenable position that accommodate all, or at least most of our members. Not easy.
One of the questions up for debate this spring has been whether photos of domesticated animals qualify as nature photography. Many of our members contend it does not. When asked why, they become uncertain but remain insistant. Pictures of domesticated animals are lumped with zoo photography as Animals In Captivity.
From this point of view, this picture is not nature photography.
Personally, I beg to differ. But I too become uncertain when trying to put my reasons into words, because there are so many lines of argumentation to follow. Most of them makes assumptions that could be subject to separate discussion, and can probably be picked apart to atoms. In the end I'm left with only one. I firmly believe that no nature photographer actually photograph nature. Rather, I hold that nature photographs are renderings of the photographer's relation to nature.
From this premise follows that all photos can be nature photos if they communicate this relation.
In the context of this particular image the sheep may look tame to a seasoned outdoorsman, and feral to an urban dweller. Likewise, the landscape may look wild or managed depending on who you are and where you come from.