You really should give those clouds some depth. They aren't exempt from lighting, nor are they a uniform thickness. Since your sky is bright, the thinner parts around the edges of the clouds should be lighter while the thicker parts remain darker. Like this:
You may also want to consider toning down the white or using a more gradual gradient, 1) because skies are not normally white unless it's a cloud cover, and 2) white would be better reserved for the highlights on the clouds, and you need a non-white sky for that to stand out.