Yes, he seemed to have at least basic street smarts earlier that evaporated here. Melanie thought he was not as smart as he thought he was, though this seemed mainly to be based on him moving his lips when he read. I think the comparative ease with which he was taken out was the weakest aspect of the film.
A later thought: perhaps he senses the police closing in and is starting to lose it. After all, there is a trail of dead bodies now. Beaumont, Melanie (OK, he didn't shoot Melanie but the cops might think he did) and Louis.
"Chicken soup - with a beep straw."

He's uneducated, but he's too street smart for that final scene to play out as it does.

It is strongly hinted that he was never that terribly smart to begin with. Certainly not as smart as he thought he was.
"facts are stupid things" Ronald Reagan

Yeah, that was pretty weak, especially considering how hair-trigger Ordell ws over walking in there in the first place.

I thought that was out of character too. All along Ordell has tended to be very cautious, and makes sure he has the upper hand in any situation.
It's even more confusing, given he has just given Max detailed instructions, that Max is going to be the first one shot if anything appears out of order when they enter the office. Yet, the office is dark, and Ordell just walks in the door and leaves Max standing there behind him. It should've been evident to Ordell that something was afoul when there were no lights on.

Ordell, by that point, was desperate and REALLY wanted his money.
Never hate your enemies. It affects your judgment.
-Michael Corleone