The light doesn't affect sharpness. However, too much light in the wrong place can appear to reduce sharpness due to bleeding.
If you take a picture of someone inside a room against a window looking out onto a sunlit scene, then the overwhelming brightness from the window will bleed into the foreground a bit. This will be especially apparent at the edges. That isn't technically less sharp, but can appear that way.
Otherwise, diffuse light can hide small variations and textures, while a point light accentuates them. The more smoothed out details resulting from diffuse lighting can again appear like less sharpness, due to the lower contrast. Again, though, the lens is still doing the same job and the diffuse-lit picture doesn't really have less sharpness. It does have lower contrast because the scene itself has lower contrast. That can appear as, or be misinterpreted as, lower sharpness.