Simply running PlanarSrf on these curves will create a correct surface.
What you are seeing is the display mesh that tries to visualize that surface.
You can modify the settings on the Document Properties -> Mesh page. If Smooth and slower is to coarse for you, you can go into the custom settings. If you have lots of objects in your scene, this can quickly lead to a large file and slow response in the viewport. If that is the case, you could consider modifying the mesh settings for that object only. You can do that in the Properties panel with the object selected.