Canon is a term derived from Biblical scriptures, refering to the authoritative scriptures that cannot be removed or changed, and in fiction terms refers to the material that 'counts'. This use for fiction was originally devised by Sherlock Holmes fans, to distinguish the original Conan-Doyle stories from those by other writers: the original stories are the scripture and anything else written is derivative.
A second meaning for canon came along in later years: when the creator or rights holder declares that part of the official fiction counts and another part does not, and which is subject to change on the rights holder's whim. For example, CBS Entertainment count only the Star Trek TV shows as canon; when The Next Generation came out, The Animated Series was effectively 'decanonised' by Gene Roddenberry's office but in 2007, CBS declared it canon again.
We should note that when Holmes fan Ronald Knox used the term "canon" to describe Conan-Doyle's work, he was satirising his fellow fans and their obsession with what 'counted' in Conan-Doyle's 'lore' by comparing them to theologians.
Official statements on canon
MTV has never made an official statement on canon for either Daria or Beavis and Butt-head, and neither has Daria producer Glenn Eichler. In an interview with Kara Wild, when talking about avoiding giving real-time dates for a TV series, said "the only purpose it serves is to get people upset about 'violating canon'", giving the impression that an official canon was not a priority for him.
Anne D. Bernstein, however, jokingly referred to canon in an interview, when talking about The Daria Diaries book: "I had to make up a ton of stuff that did not yet exist... and then it all became part of "the Daria Universe" so people had to follow what I established!". And the books written by Bernstein and future story editor Peggy Nicoll do indeed contain material that would later appear on the show itself: the names of Helen's sisters and all the details about Kevin's parents.
Something mentioned in an interview with MTV staff years after the show is not going to be canon, but will tell us about how the creators may have seen a character turning out or tidbits that they'd never fully got into the show. A number of these have been taken as 'canon' by many fans as a result, even though technically it isn't. A key example is that in his interview with Kara Wild, Glenn Eichler gave the location of Lawndale as being roughly in Maryland. The setting of Lawndale was never stated in canon and some parts of the show clash with a Maryland setting (a desert being so close for one), but since it's the nearest we'll get to an answer, Lawndale is taken to be a Maryland suburb in fanfic after fanfic. Daria's old hometown, Highland, was canonically placed in Texas in a 2011 Beavis and Butt-head, but even before then it was commonly assumed to be in Texas because Mike Judge had said that this was where he envisaged it being.
The cast made numerous appearances in interviews, MTV specials, and bumpers during episode marathons where they broke the fourth wall and referred to themselves as cartoon characters. In addition to this, there were letter pages for the Beavis and Butt-head comics, feedback on the website, and a IRC chat following "Is It Fall Yet?" where Daria spoke to specific, real-world fans. Obviously, this cannot be canon.
However, these are almost always presented in character and in some cases they were used to get across canonical information: for example, the ages of Daria, Trent, and Quinn were mentioned during Daria Day. As a result, we call this "off-canon canon" and collect it under the "Daria as Cartoon Actress" bracket.
Beavis and Butt-head
Beavis and Butt-head and its associated material is where the wheels come off a bit. This was where Daria Morgendorffer originated, and that means it's often taken as canon. However, the character was merely an occasional supporting character and was written as a foil for Beavis and Butt-head, and this means in some episodes and comics her personality is different to what we'd expect, like how Daria is utterly blase about guns being fired around her in "Incognito". Daria also downplayed its connection to Beavis and Butt-head, and any time a younger Daria was seen in flashback material she was depicted as dressing similarly to her Daria look rather than her B&B look.
Whether Beavis and Butt-head is canon for Daria, and which parts of it should be canon, is more disputed than other official material. Certainly, MTV doesn't seem to have cared!