Okay I see the number of people writing DJ Sub using “Plutonian” as an insult to most people, or hating Pluto. But I raise this:
When he calls B2J “Plutonians,” he explains (because they didn’t understand the subtext the first time he made a comparison) that Pluto was once considered a planet of the solar system, then it wasn’t, and in the same way, Rock was once a popular genre, and no longer is. He actually explains that his insult to them is that they’re a lost cause performing an outdated, “irrelevant” genre.
In other words, Plutonians is an insult specifically to B2J
He probably would come up with different insults for other people, comparing them to different celestial bodies for different reasons.
And, just because he uses Pluto as an insulting comparison in this situation, does not mean he has anything against it. In fact, he might even be able to find other qualities of the dwarf planet to craft a complimentary metaphor.