I could include this, but what would be the source? I’ve not seen it in the weather data I’ve looked at, so would it be a derived value? Perhaps using a combination of cloud cover and UV index??
Standard US weather stations do not report lux because there are too many local variables. They do report visibility, which is considered more standardized.
Over the years, some people have used cloud cover as a proxy, but that won’t always pick up fog. Or pollution, including wildfire smoke. Or a solar eclipse. 
Some individual commercial weather stations have a lux reading, but most don’t. Instead, most people use a specific sensor in the exact spot they want to measure from.
The Hué outdoor motion sensor is a popular outdoor light sensor that works with smartthings, for what that’s worth. @iquix has an Edge Driver that exposes the illumination reading: