I don't think Iran is intrinsically dumb at all. But with the photoshopping and the fiberglass its hard to mentally elevate them above banana republic status.
The difference here is that the Liechttraktor actually existed. It was a real machine, not a fiberglass model. This is not the same situation at all. Rather than actually building something for people to consider, they staged a fake, and then photoshopped it. And they did a bad job all around. It was funny. Its the sort of think where its perfectly natural to laugh. I don't understand why you aren't laughing, to be frank. Its really obvious what's gone down.
Its more a matter of the Iranians thinking we're stupid enough to buy something like this, not a matter of us thinking their stupid. Well, except in the case of us thinking their stupid for thinking something so stupid.