It depends on where we are seeing these wave fronts. In reality, the wave fronts of sunlight are spherical in nature but since the radius of these spherical wave-fronts is very large compared to what we are able to see, they appear to be plane wave front. If we are seeing from space and from a position which is far away from its generation point we will be able to see spherical wave front but when we are seeing it from earth these wave front appears to be plane in nature.