The Bell Ringer & The Royal Trumpeter
Both the bell ringer and the royal trumpeter have no trouble spotting the physical and moral dangers of the goop.
Bell ringer: "If that green stuff sticks up robins, it'll stick up people too!" (59)
Royal trumpeter: "That King of ours should be ashamed!" (68)
Together, these two represent the down-to-earth, hard-working people who run the whole kingdom, fallen prey to the whims of a more powerful figure. Sure, they may not be the brightest crayons in the box (especially not the royal trumpeter), but morally, they're as intelligent as can be. They certainly aren't destroying the environment or citizens' lives just to satisfy their own selfish needs.
Remind us again, why is the King on top?
Captain of the Guards
If the bell ringer and the royal trumpeter represent the down-to-earth, working people, the Captain of the Guard represents the lazy aristocracy. When we first see him, the dude is brushing his moustache ends. And he's so full of pride, he can't see what's right in front of him.
"Lad, are you trying to frighten me? Captains, my boy, are afraid of nothing. That stuff's harmless. I'll show you. I'll eat some." (86)
Just like the King, the Captain's need for control (I won't let a little oobleck tell me what to do!) is his undoing.
Lords & Ladies, Royal Cook, Royal Laundress & Royal Fiddlers
From Lords and Ladies to the Royal Fiddlers, the rest of the King's household is the very model of incompetence. The Royal Laundress hangs herself out to dry while the Lords and Ladies stand about looking perplexed. Together, they are the face of adult incompetence.
What would everybody do without Bartholomew Cubbins to keep everything together?