Simple
Yup: even though this is a Victorian novel with all the long skirts, horses and ridiculous gender norms to prove it, Hardy keeps it simple.
Some of the vocabulary may be difficult, but that's only because it was written a hundred years ago. The style of prose is actually quite simple: the sentences are short, and the author gets right to the point. After all, he is describing simple, country people, so it makes sense that he should use a simple style. Take, for example, the opening sentences of Chapter Twenty-One, which describe the process of making butter:
There was a great stir in the milk-house just after breakfast. The churn revolved as usual, but the butter would not come. Whenever this happened the dairy was paralyzed. Squish, squash, echoed the milk in the great cylinder, but never arose the sound they waited for. (21.1)
The sentences are short, and the author uses simple, everyday words. (Like "squish.")But of course it isn't that easy: Hardy's style may be "simple," but it's not "simplistic.
The narrator wants to show that the "simple country people" are not really as simple as city folks want to believe. The four dairymaids, for example, seem like simple, country girls, but they're all different, and they're all secretly in love with Angel. Hardy's prose style reflects that complexity when he describes them:
Their gauzy skirts had brushed up from the grass innumerable flies and butterflies which, unable to escape, remained caged in the transparent tissue as in an aviary. Angel's eye at last fell upon Tess, the hindmost of the four; she, being full of suppressed laughter at their dilemma, could not help meeting his glance radiantly." (23.14)
The sentences in this passage are longer and more complex, and the word choice more sophisticated.