Harry doesn't actually suspect McGonagall, but wishes to seem as though he does. As it's a goal that's a degree removed, he probably didn't think it through a thoroughly as he could have. Especially with the stress of other issues weighing on his mind.
Whether anyone will pick up on that depends on their faith in Harry's abilities. In general, I don't think they're so convinced that he's supersmart, and capable of maintaining composure in situations where he's under significant threat, that they'll find it unusual. Hence, his actual goal (convincing them that he needs convincing) is achieved.
So, considering those two factors, I wouldn't really consider it an idiot ball. Just Harry being human, rather than a hyperintelligent FAI.
Harry doesn't actually suspect McGonagall, but wishes to seem as though he does. As it's a goal that's a degree removed, he probably didn't think it through a thoroughly as he could have. Especially with the stress of other issues weighing on his mind.
Standard rationalization of an Idiot Ball: the character was intended to be not smart/distracted/making an error just this once. Usually author's error. Even if actually not, this argument holds too little convincing power to be worth mentioning.
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: