the sun’s rays are quite diffuse.
all spacecraft receive direct sunlight and do not burn up.
the parabola would receive diffuse rays and the rays would only be concentrated at the focal point of the lens.
only at the focal point will there be burning rays.
Several questions(actually one with several subsets). What is the temp of Mercury's surface? What is the temp of the moon's surface on the sunny side? A parabolic mirror would provide ample temp for burning things. Don't believe it? Buy a reflector telescope and then use the mirror to focus the sun on something flammable, see what happens.