Daniel Keyes's novel Flowers for Algernon is written in the form of Charlie Gordon's journal. The early (before his intelligence was enhanced) and late (after the effect wears off) entries are written in a barely-literate style indicative of Charlie's mental deficiency.
In Treasure Island, the character Dr. Livesey takes over narration of the events on ship while Jim off on the island, doing so in a very tedious style.
In Henry Fielding's novel Jonathan Wild the various crooks and prostitutes usually speak in a gentle style, but it's essentially implied that Fielding is "translating" their speech for ironic effect, and in one instance, a love note written by Wild is produced which has attrocyous spelng errerz.
Stephen King also did a version of this in his novel Misery. The main character, Paul Sheldon, spends most of the book writing a new book in his popular Misery series, and the writing style, while not really worse, is somewhat different. The format of the book within the book also changes as the character's typewriter decays — it begins in regular type, then a few letters become handwritten, until by the end the entire manuscript is written by hand.
The 'novel within a novel' is actually something of an inversion of this trope as, while the format does disintegrate, the actual story is actually a well-written, gripping narrative which the main character comes to prefer to his supposedly 'serious' writing.
Misery also has a completely straight version of this trope when Paul is writing his first draft of the new book. Paul utterly despises the Misery series but is trying to provide his "number one fan" with everything he thinks she loved about the series, including over-the-top, melodramatic dialogue and one-dimensional characters.
And he did it again in The Dark Half, where the main character's dead pseudonym comes to life and goes on a killing rampage until the main character agrees to help him write one last novel. That novel begins all right, but by the second chapter, every other word is "sparrow" (sparrows being an ongoing theme in the overall novel.)
And again in the short story Survivor Type, in which the main character is surviving through unusual means on a barren rock island. He uses heroin to anesthetize himself, and it shows in the journal he is keeping.
Mario Vargas Llosa's semi-autobiographical novel La tia Julia y el escribidor (Aunt Julia and the Scriptwriter) has a structure where intercalates a chapter telling the protagonist story with another being the argument of one of the radial Soap Operas written by the eponymous scriptwriter Pedro Camacho. While the protagonist story is told in a very consistent style, the "Soap" chapters are written in a more grandiloquent style, although not actually truly bad written. These alternate chapters detail the events going on in the various radio soap operas written by the scriptwriter, who are already very convoluted and filled with a lot of Author Appeal and Author Avatar meddlings, but become increasingly bizarre as the plots of the separate soap operas start to merge, all thanks to the increasingly unstable mental state of Camacho due to burnout.
Douglas Coupland's The Gum Thief features portions of the novel the protagonist is writing, entitled Glove Pond. It's written in a somewhat stilted style, with a bizarre plot and characters and themes clearly based on the author's own life and issues. It's sort of hilarious and pathetic at the same time.
A recurring character in Kurt Vonnegut's books is Kilgore Trout, a failing sci-fi writer. The readers are treated with short depictions of his books.
Inverted in Tad Williams's short story Writer's Child in that the main narrative is purportedly written by a seven-year-old girl using exactly the kind of style you might expect while the excerpts of her father's writing are on another level entirely. Also, the story is not comedy but horror.
Caversham Heights, the unpublished novel where Thursday Next takes refuge in The Well Of Lost Plots is described as being "of dubious merit" and the scenes we see being made bear this out. The novel's main character Jack Spratt worries the whole thing will be deleted.
A delicious bit of metafiction, as in all of Fforde's work. At the end of Well of Lost Plots, Spratt's novel is entirely changed by taking on characters from nursery rhymes. The book itself was later published in real life as The Big Over Easy. This is inspired, because The Big Over Easy was in fact the first novel Fforde wrote. After getting it rejected by many editors, he instead wrote The Eyre Affair, first of the Thursday Next books, and it was the success of this that got his original novel published as well. Not content to just publish it, though, he actually wrote it into the main series! And the real novel itself is far from bad, being widely praised, so this ends up a sort of circular subversion.
Thomas Pynchon's The Crying of Lot 49 is full of this, most notably in the forms of a cheesy family film called Cashiered (with an ending that surely caused nightmares to the fictional audience), a bad rock band who wish they were The Beatles, and an "ill, ill Jacobean revenge play" called The Courier's Tragedy, which is basically period Gorn.
The Sound and the Fury. The first section is written from the perspective of a mentally disabled man who moves in and out of flashbacks with no warning, giving no expospeak of any kind and making it ridiculously difficult to figure out the setting or the character relationships. The second section of the book is written by a student having a nervous breakdown who is apparently opposed to the sentence and constantly refers to something that he well knows never happened. Then it gets comprehensible, but the third section is about such a monstrous person that it's just a different sort of difficult.
Roger Solmes' writing style in Clarissa. He can't spell.
The Warhammer 40,000Ciaphas Cain (HERO OF THE IMPERIUM!!!) novels take the form of Cain's unofficial personal memoirs, which are pretty good. Unfortunately, because of Cain's very narrow focus (he never discusses anything which doesn't DIRECTLY affect him) the editor of these memoirs regularly finds it necessary to include extracts from other sources to fill in the context. Many of these are very poor, especially the extracts from the memoirs of Jenit Sulla, a retired general who served under Cain in her youth. The editor considers Sulla's writing to be so bad that she apologizes every time she is forced to refer to it, and regularly encourages readers to skip the extracts if they feel like it.
In the first book, information on the history of Gravalax is conveyed through a book called Purge The Unclean!, which claims to be an "unbiased" view of things. The writing is almost a parody of the typical Imperial line as expressed elsewhere in the 40K universe, and the author has a fanatical hatred of rogue traders; Vail frequently cuts off the excerpts when he starts blaming them for what's gone wrong.
This is the central conceit of The Iron Dream; Hitler is writing the novel within a novel, Lord of the Swastika, as his brain is rotting away from syphilis.
The short story "Witness for the Prosecution" by Q. Patrick is narrated by a scary, possibly slow 11-year-old whose writing is characterized by minimal punctuation and a consistent pattern of misspellings.
Isaac Asimov's story Cal features the eponymous robot's quest to become a writer, aided by having various modules installed. At first his stories are textbook examples of this trope, but the last story he writes avoids this (and pisses off his master enough that his master tries to have all the upgrades removed). As a bonus, the story is about a demon called Azazel... That's right: Cal finally became a good enough writer to copy Asimov.
In the Captain Underpants series, George and Harold write and draw (well, George writes and Harold draws) their own comics, with each book having at least one comic for a chapter. The art and spelling is, to put it simply, sub-par. Their Mirror Universe counterparts, however, draw comics which have superior art and spelling. The normal-universe George and Harold, however, think it's awful.
Both Atlanta Nights and Crack of Death were written for submission to PublishAmerica, to test their claim that they were a traditional publisher instead of Vanity Publishing. They were both written by a group of authors writing as badly as they could from a minimal outline. Both were accepted, but after the hoax was revealed, PublishAmerica suddenly backed out. Naked Came the Stranger was written for a similar, but more ambitious purpose—to see how well a "bad" book would sell if it had a lot of gratuitous sex. Pretty well, as it turns out. The working title of Atlanta Nights was Naked Came the Badfic.
The Hitchhiker's Guide to the Galaxy uses this trope with regard to the three worst examples of poetry in the universe. Subverted in the radio, print, and film versions; which give no examples of the absolute worst (that of Paula Nancy Millstone Jennings, or Paul Neil Milne Johnston, depending on the version, of Earth) or of the second worst (that of the Azgoths of Kria). The third worst, that of the Vogons, is actually included; but in a form which makes sense in-universe while being complete gibberish to the reader. Played straight in the television series, which includes intelligible examples of the first and second worst as screen text during one of the animated Guide sequences.
While stories aren't usually told in this fashion in the Discworld books, the semi-medieval fantasy setting is played realistically in that the vast majority of people are illiterate or semi-literate, and things like spelling, grammar and punctuation haven't really been standardised yet. Thus, any in-story written document is either in Ye Olde Butcherede Englishe or written like a kindergartner.
The Black Company novels are supposedly the annals of the eponymous mercenary company, and each Annalist puts his own spin on recording events. When one Annalist was sick, the Company's wizard had to fill in for a few chapters, and his writing was terrible.
Bang! Bang! Bang! Three shots rang out. Two policemen fell dead and the third whistled through his hat.
Noel's poetry in "The Story of the Treasure Seeker's" by E Nesbit, even though the other children think he's a gifted poet.
The last two chapters of the novel "Aliss", a macabre retelling of Alice in Wonderland, degenerate into a succession of laconic sentences always ending with an ellipsis. Since the book is written in first person, it's understandable that the style of writing would change after Aliss has been beaten. And shot. And raped.
In the Smoke and Shadows trilogy, the main cast is constantly beset by all manner of supernatural shenanigans, from evil wizards to demons, and the main character, Tony, has an even longer history including everything that might possibly go bump in the night. But no threat of death or insanity is worse than when their reality gets as vapid and cliched as the horrid Vampire Detective Show they all work for.
In The Pale King, part of Chapter 24 is taken from the packet of IRS orientation materials for new hires, which Wallace states is the reason for the dead, bureaucratic flavor of the narration.
In the Time Scout series, most things in the past were handmade, and most people paid attention to things like clothes and weapons. Therefor, a scout's, guide's, or tourist's gear has to mimic the imperfections of handmade equipment.
In the Agatha Christie short story collection The Thirteen Problems, each story is narrated by a different member of Miss Marple's circle. Mrs Bantry insists she has no talent for storytelling and doesn't understand how the others do it. She demonstrates this by describing the bare facts of the case in a single paragraph and saying she can't go any further without giving away the solution. The bulk of the story consists of the other characters patiently asking questions in order to get details like the names of the suspects or the existence of a Love Triangle.
A variation: In Letters to His Son, British statesman Lord Chesterfield deliberately writes in a way to demonstrate how not to write English: "MY LORD: I HAD, last night, the honor of your Lordship's letter of the 24th; and will SET ABOUT DOING the orders contained THEREIN; and IF so BE that I can get that affair done by the next post, I will not fail FOR TO give your Lordship an account of it by NEXT POST. I have told the French Minister, AS HOW THAT IF that affair be not soon concluded, your Lordship would think it ALL LONG OF HIM; and that he must have neglected FOR TO have wrote to his court about it. I must beg leave to put your Lordship in mind AS HOW, that I am now full three quarter in arrear; and if SO BE that I do not very soon receive at least one half year, I shall CUT A VERY BAD FIGURE; FOR THIS HERE place is very dear. I shall be VASTLY BEHOLDEN to your Lordship for THAT THERE mark of your favor; and so I REST or REMAIN, Your, etc." (EMPHASIS as in the original)
The short story "Voice from the Vortex" by Gareth Roberts in Doctor Who Magazine is a parody of the lousy stories in the sixties and seventies World Distributors Doctor Whoannuals, with appalling artwork, a nonsensical plot, and characters called "Dr. Who" and "Rosie Taylor" (who wears a mod dress and beehive). It also features constant glaring inaccuracies, like the time machine being called Tardis and having a rectangular console and making a beeping noise when it takes off; and writing the Ninth Doctor (a terse, witty Mancunian) with the same speech patterns as the First Doctor (a fearsome and formal old man), describing him as wearing a cloak and handbag and being chubby, and having him carry a gun and cry for no reason. On top of that the prose is riddled with malapropisms and basically ugly verbal constructions and ends with An Aesop that has nothing to do with anything that happened.
I, Partridge: We Need to Talk About Alan is made of this trope. It's the "autobiography" of the character Alan Partridge, and his writing style is sheer torture. He particularly abuses That Makes Me Feel Angry and Cliché Storm, changes tenses semi-randomly, misuses words, lies all the time and invents unbelievable embellishments, informs the readers whenever he does anything he considers clever with his writing, at one point plagiarisesWikipedia, and gives long and painfully elaborate descriptions of irrelevant details such as the exact physical way that he opened an envelope containing his A-level results. There is nothing good about his writing whatsoever.
The fictional poet Jason Strugnell, created by the very talented poet and parodist Wendy Cope, mimics the style of the great poets, but has a tin ear for imagery that results in him either using inappropriate metaphors (calling the sun "the glorious Football in the East" in Strugnell's Rubaiyat) or using high-flown phrases to describe his very ordinary life in a London suburb.
Room by Emma Donaghue is told from the perspective of a five year old boy. The novel is filled with intentionally bad grammar and sentence structure, to truly emphasize the narrator's mindset.
The Hunger Games: Whether or not the books' prose style actually sucks is up for debate, but either way, it was purposeful; the excessive usage of fragments and the occasional lack of description is supposed to emulate the narrating voice of a barely educated teenage girl in a present-tense Stream Of Consciousness fashion.
In Piers Anthony's novel Firefly; to repay in part Geode's kindness to her, Oenone tells him stories she invents on the spot. One time when she is not available he dreams she is telling him a story and it's in a completely different style, very disjointed and nearly incomprehensible. In the Author's Afterword Anthony says it was written by a man imprisoned for pedophilia.
In Drawing A Blank, Carlton's comic book of Signy the Superbad occasionally appear between chapters as he draws them. The artwork and dialog are stilted, the plot is thin, and most the female characters are improbably large-breasted, i.e. just the sort of comic book a teenager might draw for fun.
"Tight embrace of the Oak tree": holds up all romantic trops to mockery: there is a brutal macho, sarcastic ardent feminist, "sex, dr...inking, rock'n'roll" and intentional IKEA Erotica.
The Danish poet Per Barfoed (1890 - 1939) was a master of Stylistic Suck, writing under the pen name P. Sørensen-Fugholm, a fictional amateur poet and laundry owner. Fugholm's poems are goldmines of comical misspellings, tortured rhymes, Sophisticated as Hell, and unintentional (and frequently risqué) double entendres. A high point is Fugholm's homage to "Mr. Dictator Mussolini, Italy",note which parodies the widespread fascination with fascism in intellectual circles at the time whom he considers "the greatest organist of our time" for the way he and his "faksists" put an end to the "koas" in Italy. He also wishes that Denmark would get a Mussolini, because:
The parody newspaper Trangviksposten (Narrowbay Daily or something like that) was a hilarious parody of Norwegian narrowmindedness, presenting a fictional town to the east of Norway, including no less than two poets who both fit the trope - one male, and one female. The last one, called Thora Berg, who wrote under the nome de plume Bergthora, became memetic, and lended her name to a Whole genre of Norwegian "housewife poetry".