In addition to physically appearing in the Beavis and Butt-Head franchise, there are also numerous mentions of her by the two dunderheads without her actually appearing.
In "This Book Sucks", she is mentioned in a report done by Coach Buzzcut. He states that B&B had stolen her notebook and how later partiality burned pages of the said notebook were seen floating across the athletic field blown from the roof of the building after some small explosions were heard. Later Butt-Head was apprehended while holding the surviving pages of the notebook which had an essay about human sexuality (now there's your reason) and several attempts at a forged "Daria Morgendorffer" signature. Despite actually holding the pages in his hands. Butt-Head is denying any involvement.
In "EnSUCKlopedia", the duo's clay making skills include a long thin cylindrical object they "found in Daria's purse" (most certainly a tampon). Also, Daria apparently had described B&B's couch as a "Roach motel"
In "Chicken Soup for the Butt", Daria is referenced as being a Scorpio. (They tell us "Scorpios are the kinda people who are good at school and know lots of crap. They're pretty stupid.") Daria as a Scorpio is taken literally by Daria fandom but the source is Beavis and Butt-head & their summary is a bastardisation of fixed signs. It's the only source we have for her birthday though!
In the 17th issue of the comics, letter host Mistress Cora replied to fan mail - from a girl, no less - that suggested Daria needs to be less frigid and go out with one of the lads. Cora advised: "I have one word for Daria: leather! It works wonders with the boys!" (She said it does wonders for the waistline - "check me out!") This had a horrifying legacy in a later issue.
In the 22nd issue, Butt-head is asked in the letters page if he's seen Daria naked: "No but I was in Highland Hardware and saw a 2x4 -- which is the same thing".
In the 26th issue of the comics, the idiots are forced by Mistress Cora to dress up as girls. Butt-Head remarks that Beavis looks just like his mom, Beavis retorts by shouting "Shut up Bunghole, you look like... err.. Daria" (he doesn't). This is the absolute worst insult that Beavis can think of.
In the episode "Buttniks" Butt-Head's onstage "Organic" poetry includes B&B's "Diarrhea, cha-cha-cha" chant.
In the off-canon canon "Beavis and Butt-head Do Thanksgiving with Kurt Loder", Beavis said that during the final episode's wrap party "I got so drunk even Daria looked good". Butt-head tells him "you didn't get drunk and Daria didn't look good".
In a review of deadmau5' "Ghosts N Stuff" video during the 2011 episode "Drones", the lads try to remember who died at school:
- Beavis: I know Daria killed herself, I remember that. Heheheh.
- Butt-head: Uhhh, she didn't kill herself, she just moved away!
- Beavis: Whoa, really?! Wow! Heh! That's kinda surprising, y'know, I thought she killed herself.