My best guess here is that this is actually under-exposed a bit and that has caused 'colour noise'. If you look closely at the noise ... is it lots of multi coloured dots?

In Lightroom you can adjust the colour and luminence noise levels.

Sharpening this would have made it worse. This where learning to use layers would help you tremendously. Imagine this image layered on itself and you sharpen one of those layers and then only blend through the bits with no noise. In this case the sky would be less noisey.