It's actually kind of brilliant. Though he makes seemingly smart decisions, the smartest one would be to understand that he can't control anything and put GLaDOS back in charge so she could fix things.
However, instead, he makes decisions that almost cause the base to explode and lead to his own defeat. So he was still making the worst possible decision, that being to keep himself in charge, and all his following decisions and plans follow that initial bad idea.
How about when he takes you to stop the neurotoxin emission and turret production, though, eh? Eh? Eh?
And yes. Although in that case I suppose you could say that he does execute his overall (bad) plan very well. I'm sure most people died at least once after he decided to try to kill them. Now, I'd say that proves he is pretty smart at doing what he decides to do, even if it's the worst possible course of action. Loads of people do the same thing (make a bad decision and follow it up splendidly), and the only thing that differentiates them from Wheatley is that they don't always make the bad decision, because they aren't programmed to.
So I can't call Wheatley a moron, personally. He's way too brilliant at being moronic to be a moron. XD
Yes but he has no idea how to do it himself. And I guess the argument can be made that even a broken clock is right twice a day. So perhaps he's just a genius at being an idiot? I mean, his decisions only got smarter after he was attached to GLaDOS' main body.
And everyone now knows that it's her main body that drives AI mad. So one could also assume that since he has access to all the information GLaDOS has ever gathered, his bad decision-making skills were more effective and influenced by that data.
So basically, he was insanely good at making brilliantly bad decisions. Doesn't stop him from being utterly adorable for the first third of the game, though. He's one of my favorite characters ever~!
A lot of people really like him...Valve said they made the ending that way because they thought it was a fitting punishment, and people liked Wheatley too much to kill him, but I wonder what the spread really is between people who did end up thinking it fitting, people who wanted to kill him, and people who didn't think he deserved it.