Just so you know, there is an entire genre of YO MAMMA SO FAT jokes, i.e. there are hundreds of jokes that start with that phrase.

I think the joke is implying that the mother is so large that she would physically block the light, and that she would need to move to let the light shine through. 'Let there be light' is a quote from the Bible with a metaphorical meaning, but the joke is taking it literally.

It implies that the woman is so fat that she is blocking out the sun. 😅