The Onceler is the antagonist from The Lorax by Dr Seuss. “Darkiplier” is an in joke from Youtuber/Streamer Markiplier’s fans, to describe when he is acting up. Bill Cipher is a cartoon character from Gravity Falls, the antagonist from a 2-dimensional universe who looks like the pyramid on the US dollar bill.
The original poster dressed in a silly outfit and asked for feedback- presumably knowing it was ridiculous
and then (pretends to have) got mad at being compared to the character and others, joking that he’d rather be called a slur than suffer these comparisons.
The post ends with a pun on “one slur” / “onceler” which are homophonic