”se înnorează?” should be accepted as a correct answer, as ”cerul” can be understood without saying it (what else can get cloudy?)

The English sentence is unnatural. Something "clouding up" can refer to a translucent liquid which is becoming opaque, or to water particles becoming visible (for example, through condensation). An appropriate sentence would be "Is the sky getting cloudy?" or "Is it getting cloudy out?".