Emirates Non Rev Dress Code
 

How strict is Emirates dress code for nonrefundable? Smart business casual isn't the most comfortable for a long overnight flight in economy. Are they ok with solid color hoodies for staying warm and nice sneakers?

You will fit in with everyone else who is wearing their hoodies and comfy clothes back there so I wouldn't worry too much.
If you want to look slightly nicer for check in, you could wear a smart casual button up shirt and just swap it for the hoodie once on board.