Nomex shell material is like regular cloth for coveralls. Not as heavy as denim but heavier than regular cotton clothing.
It is very tough.
If you are flying a small plane and it is cold enough in the cabin for wearing long underwear then Nomex fleece becomes an option, but the regular Nomex outer shell protects against flash fires pretty well.

It is a lot less bulky, and you can just put it on over your regular clothes too.
For warmth it is like adding one extra layer of clothing so if your cabin is warm you might want to stow your regular jacket in your kit bag while you are wearing it.

If you are a passenger on a commercial flight you might want to consider a decent wool suit instead of any special clothing like coveralls.