What do you think about DoD Civilians improperly wearing military uniforms?

If it's a DOD Civilian, then they ought to wear the uniform correctly, and the chain of command, which ought to have supervisory authority, can set that policy. If it's a contractor, it's a bit harder to enforce, unless the SOW specifies attire.

CSM Darieus ZaGara explains it well.
Also, keep in mind that there are places, such as Kuwait, where DoD and DA civilians are issued some uniform items and required to wear them while working, but I have never heard of anywhere that civilian employees receive a full issue, including such items as undershirts. Likewise, unless their job specifically requires ballistic eye protection, they won't generally be issued military eyewear.

Depending on the particular job, many civilian employees will be veterans and are more likely to wear the uniform 'properly' either out of a sense of duty, pride, or just habit; but many would have never worn a uniform before and see it just as clothing they are required to wear while doing a tough job under uncomfortable and risky conditions.

As others have pointed out, there are no regulations about the details you are troubled by -- the truth is that many things we all "know" about wearing uniforms are not explicitly covered in regulations.

Your comparison of DoD Civilians who are required to wear certain uniform items to "stolen valor" is extremely inappropriate! These are personnel who have volunteered to work in harsh conditions placing their lives at risk to help you do your job, but rather than appreciate their service you want to denigrate them for trivial uniform "violations" that aren't even violations.

These DoD Civilians you are whining about are serving "with military forces" in places where combat may occur. They are required to be wearing the "distinctive uniform" because they are covered by the Laws of Land Warfare -- otherwise they would be considered "illegal combatants" despite being unarmed.

As for your "instances when I did step in to make a correction, before noticing they were DoD Civilians, and they just started going off about how I was being rude" I would suspect that they thought you were being rude BECAUSE YOU WERE! It doesn't take much to guess that those instances of trying to make invalid "corrections" without noticing they were civilians consisted of you yelling at someone from behind or at a distance. If you had walked up to the person to make a POLITE correction face-to-face, it would be almost impossible to fail to notice the large triangle insignia, especially since you should have been looking for it. Get over yourself, you aren't a Drill Sergeant and the people around you aren't Basic Trainees. If you are polite, most people will be polite even if they decide to ignore your misguided 'corrections'. But when you yell at someone, the best you can expect is to be told to go piss up a rope -- and rightly so when they are right and you are wrong.

Have some kind words with the DOD civilian wearing the uniform. While they are not a member of the service, they are perceived as a member of the service and need to look the part as in "Squared away". Is there a short form uniform guide tto refer them to?
I know USCG Aux wears the uniform of active duty with silver buttons and silver shoulder board stripes, civilians think we are the COAST GUARD so we must look the part. Uniform worn properly in a squared away manner.