Clark Clifford called him 'an amiable dunce'.
Richard Cohen wrote that 'This President is treated by both the press and foreign leaders as if he were a child.... It is major news when he honors a political or economic discussion with a germane remark and not an anecdote about his Hollywood days.' You can find various other quotations, including from the ex-President himself, used to support the idea that Reagan was profoundly stupid for a U.S. President.
Noam Chomsky rails against Reagan and the structures around him in a book from the 1990s which is a transcription of various group discussions he was involved in (I can track it down upon request.) Chomsky claims that the press were aware of Reagan's stupidity and that that is why they never sought his opinion on political matters after his presidency. From this, two side questions: is it true that Reagan vanished from the political stage altogether following his presidency and was not consulted for his view? Secondly, is that genuinely unusual in American politics? Have Carter, the Bushes, Clinton et al. been more prominent after stepping down?
A third optional question: was this image of Reagan as a dunce more widespread than it is today? Was it something people talked about often (e.g. commentators, comedians, everyday folk)?
Thanks for any and all answers you can provide to these questions, even if it's something a bit tangential.
TL;DR: Reagan was far from an amiable dunce, and while not the most brilliant man to hold the office, he was both a master manipulator and a staggeringly effective manager.
Just a few points (as this isn't my area of speciality, and, full disclosure: I have a negative overall view of Reagan), and sources if need be.
First, every skilled President effects certain personality traits. Eisenhower was well-known to his advisers for pretending to either be confused or confusing others when he did not want to give a straight answer. Indeed, it took until archives opened up for the popular opinion of Eisenhower to shift from amiable near-dolt to calculating tactician. With Reagan, although he was very much known to veer into Hollywood storytelling or homespun digressions, a great deal of that was a disarming tactic. Did Reagan also clearly have some seemingly ridiculous ideas? Absolutely. But an idiot is easily outmanoeuvred, plans poorly, and cannot manage people. Jimmy Carter was a brilliant man, but also one of the worst presidents when it came to delegating and avoiding micro-managing.
Reagan managed some of the most intelligent, devious, and in some cases crooked individuals in existence, and yet somehow had low turnover in his cabinet, avoided any real consequence over Iran-Contra, and gained the sobriquet 'Teflon President'. My point here is that while there is a great deal to dislike, there is simply too much evidence that he was a master-manipulator (if a half-rate actor) to dismiss, and to credit all the successes of his administration to others while attributing the failures to Reagan is simply inappropriate.
Part of the reason Reagan "disappeared" from the political stage by, say, 1994, is that his Alzheimer's was becoming a genuine issue for him. From my personal experience with people suffering from dementia and Alzheimer's I believe one can see this in Reagan's later years in the White House, but obviously I'm neither a doctor nor did I know the man (There are sources that support this position Lesley Stahl's "Reporting Live" for example). However, he was actually fairly active until that point, but he advocated more for domestic/administrative changes than anything intellectual (Brady Bill, repealing the 22nd amendment etc.).
I would also point out that Noam Chomsky is far from the least biased source. Taking a linguist who has viewed America's foreign policies as neo-imperialism for decades as a fair source is quite problematic. Noam Chomsky loathed Reagan's policies and thought they were morally repugnant (I'm presuming he still does).
While this doesn't mean necessarily that his criticisms are therefore baseless, it is nonetheless hardly convincing for someone to say "I think everything you've done is wrong, predatory, and evil, but I admire your intelligence." Basically, I don't think it is a surprise that an anti-interventionist, anti-imperialist, anti-capitalist would find a great deal to dislike about Ronald Reagan. To simplify, for Chomsky (as Fred Halliday once quipped) "Everything America does is wrong", but for Reagan America was the genuine manifestation of Right. So, if you can find some evidence from George Bush, Weinberger, Thatcher, or say, Robert Dallek, that says Reagan was stupid, I'd give that more consideration.
The final question you asked simply allows me to share a bit from impersonator Rich Little, which he came up with during Reagan's presidency. The image of Reagan as a well-meaning but ultimately dim and easily dominated old fool was quite prominent, especially amongst left-leaning folks during his presidency. Little's bit went something along the lines of Reagan apologising for being late to an event, but something came up an Nancy wouldn't let him out of the house. The joke played both upon Nancy's infamous astrologer Joan Quigley, and the fact that Nancy Reagan often played a more prominent role in Reagan's day-to-day life than previous first ladies.
So rather than blather on I'll simply state again that while I am not a huge fan of Reagan, or many of his policies, I do not doubt for a moment that he was a very intelligent, charismatic, and effective leader - idiots can't do that.
Sources:
Michael Schaller's Ronald Reagan (2011)
Martin Anderson's edited volume Ronald Reagan : Decisions of Greatness, (2015) - I am fully aware of the rather obvious bias.
Robert Dallek's Ronald Reagan: The politics of Symbolism (1999 version)
and David Bosworth's article "Sunny Jim: The Genial Demagoguery of Ronald Reagan" in Salmagundi - Fall 2014.
Related question: Reagan died of Alzheimers. I've heard that he was affected by the disease during his presidency. Is there any evidence that it affected his mental capacity during that time period?
As a follow-up, what was the perception of him while governor of California? I ask because we're definitely getting more solidly into pre-alzheimers years, and I seem to recall some bits from Laugh-In reruns suggesting he wasn't very bright, though I could be misremembering those.
was this image of Reagan as a dunce more widespread than it is today? Was it something people talked about often (e.g. commentators, comedians, everyday folk)?
In support of this point, I'd introduce this piece of evidence: The Wit and Wisdom of Ronald Reagan, a 1980 LP that consists of blank grooves. It looks like it should be playing something, but it's silent.
1980 is also when the comic strip Doonesbury went on a wildlife excursion "In Search of Reagan's Brain" (once inside it, intrepid reporter/commentor Roland Hedley discovered he was "only able to see backwards through a rose-colored mist"). Hedley returned in 1987; some of those strips are here.
So yeah, this was a running joke going into his presidency - he was inaugurated in 1981.
It might have made a convenient cover for being a master manipulator; it might have been the truth. My suspicion is that he got the rap because his most popular movies co-starred a chimpanzee (and the chimp's name was bigger on the posters), and because he was a little goofy.
Not dull... but a little old fashioned, and the kind of guy who'd see an open mic and say, "My fellow Americans, I'm pleased to tell you today that I've signed legislation that will outlaw Russia forever. We begin bombing in five minutes," without foreseeing some repercussions.