At first I thought this was a pejorative, the Centauri trying to insult G'Kar even more; but the more you watch, the more species you see calling him Citizen as if that is his official title.
What I don't understand is why they don't just call him G'Kar. Why stress "Citizen"?
G'Kar was no mere ambassador: he was a member of the Third Circle of the Kha'Ri, one of the twelve most important Ambassadors of the Narn. He proved himself in this role to a high degree, showing great effectiveness in leading the Narn resistance against the Centauri, to such an extent that he was specifically targeted for removal in any way possible.
The Centauri went out of their way to call him Citizen G'Kar to denigrate him, to emphasise that he was not important, and to remove him from the position of power and leadership he still retained. Their intense efforts did not go unnoticed by the rest of the Narn resistance, however. The Narn took his non-title of Citizen and turned it into a celebration; this was the only Narn singled out for the Citizen treatment by the Centauri, and therefore Citizen G'Kar was the most important Narn in the galaxy.
In 'The Long, Twilight Struggle' Londo makes it clear that G'Kar is no longer eligible to represent his people as he is no longer an Ambassador but merely a Citizen of the Centauri Republic
"...as a result, Ambassador G'Kar may no longer represent the Narn in any official capacity whatsoever. His appointment as ambassador to Babylon 5 is hereby withdrawn. As the only member of the Kha'Ri still at large, Citizen G'Kar will return to Narn for trial." (Emphasis mine)
After this, the Narn increasingly use this as a semi-title, intentionally mocking the Centauri by continuing to defer to him in matters of importance and using "Citizen" as his title.
Holding the title and privileges of Ambassador indicates G'Kar must perform specific responsibilities as a political figure. Although he is stripped of his rank and mocked as a Citizen, the title is reclaimed.
As a Citizen, he lives both within the Ambassadorship and outside it. He is able to say things that as an ambassador he may have to refrain from as he was within the political circle. As Citizen G'Kar, he becomes an anomaly and as such is both accorded respect of a previous title and can speak wholly for his people. He is a Citizen and as such, he represents every citizen without a full title. His words become more powerful and the mockery is turned against those who would deny him his Voice and the Voice of his people.
A different interpretation would be that the citizen highlights the lack of any other title. "G'Kar" might be an informal way to talk about "Ambassador G'Kar". "Citizen G'Kar" isn't.
