The type of people you encounter in the US military depends on the work you are assigned, and whether there is a war going on. The dumbest soldiers I encountered were in supply, the motor pool and a few infantry types who thought anyone who wasn't on the battlefield 90% of their TIS wasn't military.
I was a Broadcast Engineer for AFRTS, worked on the WeatherVision system at Ft. Rucker, and installed some sound systems. I tested out of the three year electronics school at Ft. Monmoth and was awarded my MOS while in basic. I was told that almost no one passed that MOS test, even five years after completing the course. I received the highest score on record for my MOS at Ft Knox.
The people I worked with were no different than in any other technical job. It was the unhappy people doing mostly menial military jobs that didn't seem very bright, but some of them were just bored to death. If you got to know them you discovered that some were well read, and some were taking one or more military correspondence courses, or going to night classes at a nearby college. Others would have been losers, no matter what they did, and a few were in the military to keep from going to prison.
If you served during W.W.II or Korea, they took a lot of people they wouldn't have, in peace time. They needed people who could shoot, and who could be quickly trained to take care of themselves on a battlefield. In a lot of cases, they were strong, but not well educated farm boys. People who couldn't afford college, and spent most of their lives doing hot and heavy labor on the family farm. They wouldn't have scored high on an IQ test, but in no way were they stupid. They just never got the chance to get much of an education. OTOH, they were the guys you wanted at your back when the shit hit the fan and the fighting meant life or death.
You see more deadbeats in peace time, because there is no fighting to send them to. It's very easy to transfer someone to the infantry, and a war zone if they cause too many problems. Almost everyone soldier has infantry as they primary or secondary MOS. Very few have 'NA' for a secondary MOS on their DD-214.