I might wear my MarPat boonie cover just cause its functional, and weather permitting, I would wear a PMI jacket if I had one. For the most part, I try to wear everyday clothes at the range, but I might opt for long pants even in the heat if I'm going to shoot low kneeling or sitting.
I honestly don't think there is much that is inappropriate for the range. If some kids next to me are all decked out in NIN shirts and trenchcoats, and acting stupid, I might keep an eye on 'em, (But I'd keep an eye out for saggy shorts/gang apparel or dinnerplate belt buckle types acting stupid, just the same.) I suppose if someone showed up with all their Airsoft Mall-ninja gear I might be concerned, but if they weren't acting like total idiots, who cares. Sure, apparel can be a warning sign, but I have met plenty of people who dress like fools, and still act concientious and professional.
I'm former military but only wear cammo while hunting, not in public places even if it be the range.
Yeah, it still bothers me when I see guys wearing their cammies in public. It's completely irrational, (Not wearing utilities off base except for commuting is mainly a Marine thing,) for me to impose my ideas on other servicemembers, but I still do it. And most former military I know are reluctant to get all dressed up, (Outside of a single favorite item,) so when I see someone doing it outside of a military context, I immediately think "Wannabe". But once again, their behavior is far more important than their dress.
As others have pointed out, maybe they are training for their job, though most places/bases I have been, training like that is conducted on military/LEO exclusive ranges, or at least during military/LEO exclusive times, and always in groups, (AmShootCen in San Diego does LEO training in the morning before they open to the public, for example.)
I certainly have never been to a range that had any sort of noticeable or enforced dress code, but then again, I have only shot in CA, AZ, and Guam.