You can change your terrain to a lower HeightMap Resolution like 33, 65, 129. The lower the heightMap resolution, there are less points to interpolate in the mesh.
You could change the Pixel Error as one of the answers states but increasing the Terrain Pixel Error can lead to other side effects since it affects LOD. For example, when increasing the Pixel Error too much, you will see your terrain reshape as your camera approaches it.
This explains more on the pixel error.
In short, been able to get the best results for using a LOW pixel Error value and a low heightMap resolution for a low polyish look for terrains.