I would simply mention the lack of insulation (depending on where you are located) and if it SHOULD be present reccomend that it be brought to an adquate level for the climate/area in which it sits. If you are in an area where such is not uncommon, fergiddit, other than to make the report that it isn't there.
T'aint your job to require anything code related, except actual health and safety issues. Just report the lack and let the underwriter make the call.
Background is that the old FHA system and forms required you to indicate adequate, inadequate or non-existant. They tossed the baby out with the bathwater and now for the most part all you should do is indicate if it is present and how many inches there are.
90% of homes in my area dont have insulation in the crawl space what so ever. We do have cold winters. No insulation no prob. There was a thread a while back about palstic on crawl space floor you might try to locate it. Not a big deal usually.