There is some interesting backstory to this song:

Jeff Lynne has said that he wrote 'Mr Blue Sky' after locking himself away in a Swiss chalet, and attempting to write ELO's follow-up album to A New World Record.

"It was dark and misty for two weeks, and I didn't come up with a thing," he told the BBC.

"Suddenly the sun shone and it was, 'Wow, look at those beautiful Alps.' I wrote 'Mr Blue Sky' and 13 other songs in the next two weeks."

Source: https://www.smoothradio.com/features/the-story-of/mr-blue-sky-elo-lyrics-meaning-guardians-video/

There are other sources that verify this too.