Prince Bob dresses in a fashion that is so bizarrely Spartan that it is itself a form of showing off. No real begger would ever wear anything like that.
He was kidding when he said that.
That body being of course, a state of the art Mobile-Suit Human, which is designed to be ordinary looking, so people can judge him based on his actions.
Parodied: Bob looks like a monk.
Zig Zagged: Some royals wear ornate clothes, others dress more casually. Bob's mother is likewise sensible with her outfit but his sister wears too much jewelry to move.
Averted: Bob dresses about the way you would expect a Prince to look.
Enforced: It's a stage or dance production where Requisite Royal Regalia is too expensive or cumbersome for the character — they dress Prince Bob plainly and use minimal costuming and dialogue cues to show who is the ruler.
Lampshaded: "I'm surprised anyone takes Bob seriously as a ruler, given the gym shorts and baseball cap he's always wearing."
Invoked: Bob dresses in simple clothing to show that he is a selfless leader who won't spend extravagant sums on himself.
Defied: "By my crown, I will not be mistaken for some lowborn peasant!!"
Discussed: "That's the Prince? I thought he was the gardener!"
Conversed: "So, which one is the King again?"
Deconstructed: Bob dresses plainly because no one, not even him, takes his title seriously. The monarchy is a complete joke and the ruler is just a figurehead. Consequently, when he is called upon to lead in a crisis, no one will listen to him.
Reconstructed: Bob dresses plainly because he is not only a pretty face but a competent leader, and he doesn't need showy clothes to command respect.
Back to Modest Royalty, your Highness, but please...do something about that...tee shirt, was it? What would your mother, the Queen, bless her soul, think!?