He didn’t say that it’s cloudy, he said “the sun isn’t shining” which could also include nighttime.
There are aircraft that use solar power, such as the NASA Pathfinder.
It has a backup battery to handle conditions where there is inadequate sunlight, and of course it’s unmanned. I’m not aware of any plans for manned aircraft to rely on solar power.
Trump does this often. He hears something, comes up with a potential problem based solely on speculation, then acts like this is some issue that he came up with himself because he’s a genius that none of the “experts” have thought of or are choosing to ignore. He loves him a good strawman, it’s easy for tiny hands to push over I guess.