The only story even close to that description I've ever read is about a WWII veteran taking a highly advanced correspondence course.
With integral calculus in the third lesson.
This story is not a cautionary tale against being too knowledgeable or too competent -- it is difficult to imagine science fiction authors being anything but enthusiastic for as much knowledge and as much competence as possible.
So many of the "If This Goes On..." style of warning stories warn of too little knowledge and too little competence, or at least without wisdom and ethics to use it well.
But in the story I know closest to the question, an almost
"exactly average man" decides one day to buy, from an ad in the back of the yellow pages or newspaper classifieds or similar, a set of lessons that claim to impart a particular skill.
He is a seriously injured, and widowed, veteran sent back home in 1945 who, in response to a letter, starts taking a correspondence course.
It is (of course!) "Correspondence Course" by Raymond F. Jones.
He is an extremely sympathetic character because of his losing his wife, and because of his war-shattered leg.
He starts the correspondence course in response to a letter:
SERVICEMAN—WHAT OF THE FUTURE?
You have come back from the wars. You have found life different than you knew it before, and much that was familiar is gone. But new things have come, new things that are here to stay and are a part of the world you are going to live in.
Have you thought of the place you will occupy? Are you prepared to resume life in the ways of peace?
WE CAN HELP YOU
Have you heard of the POWER CO-ORDINATOR? No, of course you haven’t because it has been a hush-hush secret source of power that has been turning the wheels of war industries for many months. But now the secret of this vast source of new power can be told, and the need for hundreds, yes, thousands of trained technicians—such as you, yourself, may become—will be tremendous in the next decade.
He does not learn widely varying things like piano and bodybuilding, but does learn the technology of the "power co-ordinator". And wonders how even the best scientists on earth have invented all this just since the war.
As you would be guessing while reading, scientists on earth haven't.
He takes two trips to a town that says "Population 806" and wonders if the population had declined since the sign was made, before finding the installation of the "business" offering the correspondence course, and promising some sort of opportunity.
It ends in a way that could be called "sadly" for such a sympathetic character. But he is not punished for hubris, or for being "too" knowledgeable and competent, as could happen in mid-century middle farmland America.
It's not a warning to be careful what you wish for.
He wasn't punished; he was needed.
So while it did "[end] in disaster for the average man", this particular story is not "a cautionary tale about excesses in any form."
It's an Alien Encounter story.
And if the aliens are truly alien, nowhere near the Uncanny valley in a way that makes them sympathetic to us, then the two races might be too incompatible to benefit each other.
There is some duress and threat of force, and he could be harmed, or die before anyone else found him. It looks disastrous to human readers, including me, as he has a completely alien experience at the end.