Paleontologists deduce the eating habits of extinct animals by analyzing the shape (morphology) of their teeth. Consider how we use our own teeth: The front teeth (incisors) are knifelike and used to tear large pieces of food (such as an apple) into manageable chunks, and the fangs (canines) act like stabbing knives. The back teeth (molars) are relatively flat and used to crush chunks of food into a digestible mass. Mammalian predators tend to have more prominent incisors and canines than we do, and their molars tend to have sharp ridges that allow the opposing teeth to act like a pair of scissors rather than a mortar and pestle; they bolt their food without grinding it. Herbivores tend to have more prominent molars that allow extensive grinding to release nutrients from tough leaves and other plant matter. Humans are omnivores with a relatively versatile set of teeth.
We can apply these concepts to Yoda's species, but the results are somewhat inconsistent, as shown in this selection of photos.
His teeth look very human, if well worn, in most scenes, with visible incisors and canines in the middle photo; we can't see whether molars exist but the shape of the jaw suggests that there are additional teeth. Yoda could be an omnivore based on these photos. In the left scene his visible teeth look triangular and sharp, clearly the mouth of a predator, but this scene doesn't seem to be from the main series of films. As Valorum points out, Yoda has also been seen making a vegetable stew, so he eats plants at least. Conclusion: Yoda's species is omnivorous but Yoda himself may practice vegetarianism.