Actually this is not pixellation due too low resolution. This is a result of strong JPG compression and heavy noise removal & sharpening in your camera or photo editor.
If you have the raw image (not *.JPG, but *.RAW or *.CRW), you should learn how to develop them properly. I think you have only the JPG, but something can still be done i.e. your case is partially solvable. The idea shortly:
- Make a clean BW copy of the image where the sky is white and the wires, stakes, houses etc. hard things are black, the grey areas are minimized
- Blur the colored image without trashing the edges of the objects (see it later!)
- use blending mode multiply to make black solid
The key is Smart Blur. Gaussian blur makes everything fuzzy. Smart Blur leaves local contrasts which are stronger than the selected treshold. That function hasn't unfortunately in my old Photoshop full screen preview, only a small preview stamp in the dialog => I cannot show the under construction screenshots with the result. I show only the results.
Make some test to understand the settings of the Smart Blur. Note: Blur radius and Treshold interact, they aren't independent. That makes the setting tricky. You must test many combinations!
So, after you have the copy in the top layer, goto Image > Adjustments > Hue/Saturation and with quite wide selective adjustments make cyan-blue-magenta-red as light as possible:
Increase the contrast with the curves. Beware making the grey between the lines dark, but reach as white sky and as black solid things as possible. Some grey is unavoidable.
Then apply Filter > Blur > Smart Blur. Play with settings to make the clouds soft, but not fading the thin wires.
After blurring smartly: Fix the mid sky grey area. Paint white onto the grey. Leave the near horizon greyness or remove it all (a hefty job between the wires and stakes, needs small steps and much patience).
Here's the result:
Apply Smart Blur to the colored image layer. Give to the BW layer blending mode = Multiply.
Here's the final result:
If the sharpening (it's already in the original image) starts to look out painful, you can wipe it off with the clone brush. Do it in the colored layer, the BW layer save the sharpness. To remove the remnants of the sharpening in the area where the BW version has grey (=mid bottom)unfortunately needs also removing the grey.
An opinion: You can as well let the image be as is or apply even some arty filters. My receipe nor other heavy processings do not change technically weak material perfect. High technical image quality is born (or killed) already when the shot is done.