Of course, the only ones MAKING that assertion are Londo's people, who have much to gain in trying to convince others of that.
For a thousand years, the Centauri Republic was a force to be reckoned with. Like the English empire once upon a time, it held hundreds of planets in its control. It was a great military power. But slowly, as can happen, they grew content, and lazy, and gradually their own empire began to slip between their fingers. A world deciding to go rogue was troublesome, to be sure, but it's SO far away, and it's SUCH a bother to go take care of it, when we can easily get the same things from other places...let them go. They'll come crawling back sooner or later.
As a result, they are now down to a Republic that consists of barely a dozen systems and thirty worlds.
It was, interestingly enough, the Centauri Republic that was Earth's first contact with another major government. The CR was well in advance of Earth science, and we all considered them a terrible power...an illusion they hardly tried to set right. Trade agreements were set up, and we gained an ASTONISHING amount of technical know-how in a very short time, letting us leap- frog a hundred years of progress in a single year. They were most curious to get cultural stuff in return...music, art, philosophy, literature..."native" trinkets that could be resold for more money back on homeworld.
In the thirty or forty years since then, however, we've found out the truth, that the CR is really on its last legs. And we've taken the technology we've gotten and perfected it, and now the Earth Alliance is fast becoming one of the dominant forces of this time. And the Centauri Republic is trying to attach itself to us the way a ramora attaches itself to a shark...for preservation, in this case.
They are governed by an emperor, and the government works mainly through personal and family influence. It's a very indulgent society, and Londo reflects that. Overweight, prone to gambling constantly (null-pool is his favorite), and fond of women and drinks, he understands his role and doesn't try to push it. Like his Republic, he subsists on old stories and tales of former glory, remarking -- one night, when drunk -- "my god, we've become a tourist attraction. See the Great Fallen Centauri Republic, open nine to five...Earth Time." He is, by turns, a comic figure, and a tragic figure.
Certainly, you could wear your hair longer than your status permits, but it's like pretending to a status you don't have, which is viewed as pathetic.