Hey, I'm OP. Below is the intended solution, with my own logic for all the clues. Congratulations to User SmarthBansal and additionally to User David Robie.
"The Lick", an infamous jazz cliché melodic phrase. The most common set of notes used as an example of this is D E F G E C D, but this can be transposed into all 12 keys and used in many contexts.
"If you have to ask what the answer is, you'll never know!"
The title, as David Robie noticed, is a reference to a famous quote by Louis Armstrong, famed jazz trumpeteer ("If you have to ask what jazz is, you'll never know!"). This is the way I figured that you guys would be able to narrow the topic down to jazz culture.
A pattern of lows and highs
The lick, as a musical phrase, obviously contains "lows and highs".
In groupings of ones, twos, and fives
A clever (if I do say so myself - and this is my post, so I do in fact say so!) reference to the common ii-V-I jazz chord progression, ubiquitous within the genre and one of many possible harmonisations of "the lick".
And when I appear / The masses may cheer
Really, the lick sounds pretty good if you've never heard it before. It can be used pretty well, and it sounds nice,
But those in the know roll their eyes
Any jazz musician worth their salt rolls their eyes at anyone playing "the lick", because they've heard it so many times that it's become hackneyed and is no longer original. It can still be used as an ironic joke, though, as evidenced by a lot of the more popular music channels on YouTube right now :)
0 + 2 + 1 + 2 - 3 + 0 - 4 + 2
This is another numerical representation of the lick, but starting from a root note, it represents the number of semitones between each note. The 0 in the middle was supposed to represent the rest, since no semitones change on that beat of the lick.
Or, in plain English: Figure this one out, you @#$%#!@
As observed by Smarthbansal, @#$%#!@ is 2345312, which is "the lick" in scale degrees (starting on the supertonic degree, because that's a more common representation of the lick, for whatever reason).
Also, I know I said the clues were only above the line, but I'd be impressed if anyone had
checked my user profile and seen that my highest-rep SE site is Music.SE,
deducing that the topic could have been music.
I realised after the fact that that was possible, so if you did that, kudos to you.
Bonus image: I made this from the knitting part of SE last winter when we had seasonal hats!