These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
MADtv did a parody of the MTV reality show, The Ashlee Simpson show, which included a sequence where Ashlee Simpson's lousy singing is remedied by sound engineers making her sound like a mix between Britney Spears, Jennifer Lopez, and Paula Abdul. While Ashlee is lip-synching to the track, she momentarily stops to groove to the music, then stops when she realizes she's not lip-synching. About two years after that sketch aired, Ashlee Simpson would actually be outed as a lip-syncher on MADtv's rival show Saturday Night Live. (See also above.)
The "Sopranos Censored on PAX" sketch — back then, it was a commentary on how The Sopranos' violent, sexual content would be reduced to nothing on a channel that wasn't HBO (which was gaining fame for showing television shows that pushed censorial boundaries). These days, this joke has become a reality (only The Sopranos would be Edited for Syndication on A&E, not PAX. In fact, PAX stopped being a channel in the mid-2000s and was replaced by a superstation called ION).
The Friday the 13th: Jason Takes NASA sketch is now a reality, thanks to Jason X.
One of the sketches they had during the 12th season was an animated three-part sketch showing pilots that Cartoon Network rejected (one show about an amputee superhero team and another about a Korean superhero who decapitates people). Cartoon Network (and its parent company, Time-Warner) were the ones who acquired MADtv and turned it to MAD after FOX gave up on MADtv and canceled it.
One of the sketches for "Clops" had Gumby running away from the police and robbed a grocery store. A similar event happened in 2011 where a guy dressed in a Gumby costume decided to rob a grocery store.
Shallow Parody: There's a parody of The Dark Knight made during the final season where Batman (played by Matt Braunger) couldn't afford good gadgets because of how bad the American economy has become and make it seem like that without gadgets, his villains could easily kick his ass. While it is funny, any Batman aficionado will tell you that Batman portrayals haven't been especially gadget-heavy since the campy 1960s TV show starring Adam West, and, while, Batman still has the coolest toys in later incarnations, he relies mainly on his detective work and physical abilities. Though this case of Shallow Parody is undercut with some Rule of Funny, as the sketch more made fun of the fact that the American economy is in the crapper and a lot of once-rich people are trying to be inventive and resourceful in laughably bad ways.
They Changed It, Now It Sucks: Much like Saturday Night Live, a lot of fans have cited many cast and writer changes (and a change in studio and show format in its last couple years) as a reason why the show went downhill, as well as the show being based on MAD Magazine despite that all of the magazine's hallmarks (Alfred E. Neuman and his "What me worry" catchphrase, the Don Martin shorts, and the "Spy vs. Spy" cartoons) were phased out after season 3 and the show became a cheap knock-off of post-1970s SNL mixed in with In Living Color! in its fourth and fifth season and the short-lived pre-MADtv sketch shows, House of Buggin and Saturday Night Special.