At the start of the second volume Order of the Stick, No Cure For The Paladin Blues the plot to date is summarized by Elan, using finger-puppets.
A Punch an' Pie subplot includes three pages out of a So Bad, It's Good novel by one of the characters. It's written to be received as So Bad, It's Good by the readers in the real world. The general public (or at least the critics) in the webcomic world actually think it's a good novel. Justin (the author) and Angela think it's crap.
When the author of Brawl in the Family decides to take the day off and lets Dedede draw a comic, it has has loads and loads of this.
The authors of Erfworld created a site for Parson's "Hamstard" webcomic. Suffice it to say that "Nobody reads my webcomic" is one of his laments about his real-world life prior to his summoning.
The works of Tycho's archnemesis L.H. Franzibald in Penny Arcade, whose undeserved success is a great source of jealousy for Tycho. Gabe, of course, is immediately hooked.
"I am Grimm Shado," said Grimm Shado, his triple wand claws extending. "And I am here to take it to the limit."
And Franzibald himself is an offshoot of the "Elemenstor Saga," an "epic" fantasy series about wizards and talking furniture supposedly written by the Tycho character. In an inspired bit of meta-metafiction, Gabe and Tycho created a wiki where readers can create their own continuity for the fictional works.
The theatre and TV shows the characters put on in Something*Positive (such as Nailed!, a musical version of the Crucifixion), all appear to be terrible, especially Aubrey's TV series My Neighbor Cthulhu which was so bad the State of Massachusetts served her with a restraining order keeping her away from TV production equipment.
Also Davan and Jason's Cat Girl comic strip Neko Neko Holy-Chan, which Davan holds in such contempt the idea of meeting the people who like it fills him with horror.
Shortpacked!!'s Amber has written at least two blatant Mary Sue stories. Her superheroine, Amazing Girl, was impervious to criticism and had no father issues. Her online romantic novel, in addition to being a hilarious Twilight parody, is an extended metaphor for her ideal romantic life.
The various stage performances seen in Girl Genius are as overblown and corny as possible, featuring all sorts of hackneyed mad science and adventure cliches, grandiose romances, and even more grandiose dialogue. An opera called The Storm King is a particularly "impressive" example.
Most of the main characters in Achewood keep blogs, and the some of the more dysfunctional characters have atrocious blogs. And the dangerously psychotic Nice Pete has made two forays into the world of novel-writing, both of which manage to be talentless and extremely creepy at the same itme.
"When I want your opinion I will cut out your brain and eat it and crap your opinion back into your skull"
Peanut of Housepets! writes his own comic featuring Spot (the Superdog), drawn in a crayon-and-lined-paper style, and written in an exaggeratedly amateurish style, with a Boring Invincible Hero, who is also an Author Avatar and delves into the realms of bad Fan Fic at times.
The hilarious part is that it takes actual effort to make it as crappy as it is on Photoshop, because MS Paint can't actually be that low-quality in and of itself and merely using the lowest JPEG setting on Photoshop isn't enough. Andrew Hussiehas described his techniques for reaching that level of graphical shittacularity as being fairly intensive and detailed.
Mindfang's journal is written in an awkward, fanfic-like style. Unusually for a fanfic parody, it spoofs "good" fanfic - literate, creative, but badly-worded and paced and with lots of jarring sexual content.
Karkat cannot be conveyed with a more detailed portrait yet. He is too angry, and is forced to look like shit.
Caliborn is the epitome of this. While HOMOSUCK, his dramatic retelling of the story, is miles better than his original drawings (Which were little more than scribbled lines), when combined with his immature misogynistic writing style, you get something that turns the trope up to eleven... thousand. Lampshaded by the acronym for Dave's planet in the second act, LOSHIT.
Irregular Webcomic! uses a fictional version of William Shakespeare, where "Will" is a modern-day office worker instead of the historical figure known in Real Life. Instead of writing epic poetry and plays, he focuses his time on writing Harry Potter fanfiction, which is portrayed as being low-quality writing chock full of wish fulfillment of Will's fantasies. The very idea of Shakespeare writing fan fiction would likely fit this trope, even if Will's writing didn't suck.
Marigold's fanfiction... could use some work. Jeph had a lot of fun writing that.
Also, Jimbo's romance novels.
Gunnerkrigg Court has a bonus comic called City Face. Its black-and-white art isn't bad per se, but it's certainly much simpler than the highly detailed and painstakingly colored art that Gunnerkrigg normally features. The dialog has a unique flavor (i.e. highly awkward and stilted, but apropos for the characters in question), and the overall effect is... interesting. The comments in the Shout Box below each strip are part of the joke—written by the author and patterned as a mockery of internet flame wars. Strangely, despite the odd style and superfluous plot, the City Face storyline (and its sequel City Face 2) are are stated to be canon by the author.
Super Mega lives and breathes this trope, in bright yellow.
Puppy on top of other animal: "Where did this unicorn's horn go?! Is the magic gone forever???"
Power Up Comics is meant to be a jab at those who create really sloppy webcomics by cutting and pasting a bunch of canned drawings into simplistic backgrounds and fill it with punchline-less jokes or lazy writing.
Isaac Baranoff did a one-off comic strip mocking a fellow cartoonist who he felt was "biting" his humor and art style. Baranoff's parody of that cartoonist was intentionally done as a bad stick figure drawing, mocking the rival cartoonist's style, which Baranoff disliked, and the dialogue basically accuses the other cartoonist's Expy character of being a Mary Sue.
Subverted in Axe Cop. The entire series is written by a 6 year old (literally), but the good artwork by the writer's older brother brings it up to awesome.
PvP's Marcy used her webcomic i hate your face as part of her application portfolio for art college. The twostrips shown were rather dire.
In El Goonish Shive, Elliot and Susan's review show has a scene where Sarah throws a couch at Elliot. In universe, the characters wanted the couch graphic to look obviously fake, so that no one would take it too seriously. Out of universe, this meant that the author did such things as deliberately failing at perspective, re-using the same couch graphic, and making it change size between panels.
Nebula: At one point Jupiter has a daydream about what things would be like if he were in charge, after he finally succeeded in killing Sun. Since this is Jupiter's daydream, it's of course shown with scribble art and filled with really clumsy out-of-character dialog from the other characters who appear in it.
Similarly, rage comics don't exactly have a beautiful artistic style, but they provide a framework where artistic rank amateurs with a sense of humor can compete with people with artistic talent on equal ground. So the suck is not so much stylistic as it is functional. Sadly, most post-2010 rage comics tend to use less original art and instead copy used facial expressions and play on common high school humor.
Polandball comics, as a rule, are supposed to look like they were drawn with an unsteady hand using a touchpad or computer mouse on Microsoft paint or something similar. The characters are simple balls with a country's flag and two pupil-less eyes (the more complex flags will still be drawn by a touchpad or mouse). Using tools to make perfect circles or copying and pasting a hard-to-draw emblem or symbol from a flag (besides for the purpose of getting the right colors for it) is heavily frowned upon. Still, artists who have more advanced art programs do sometimes manage to make lush illustrations while still following all the rules.