"ACAB" is a meme phrase on Tumblr and similiar social media circles, it has as much weight as the "trans rights" and "eat the rich" mantras because the only real-world knowledge the speaker has of what they're parroting is from biased text posts and shitty memes they bounce around the Tumblrsphere... which they consider "activism" and proof of how "punk" they are. They're also huge consoomers of media, so they end up slapping these phrases on fictional characters to appeal to the other Tumblrites, because highly-parroted political phrases + recognizable character = relatable!!! = likes and reblogs.