Q: In Gundam UC, Minerva is referred to as a princess, but Zeon was a principality (or duchy, depending on the translation). What gives?
A: Duchy/Principality of Zeon is a mistranslation.
There is no real
translation. In real life 公国/koukoku means duchy/principality, yes, but this is because in real life the 公/kou comes from 公爵/koushaku = duke (principality comes from how in French a prince is also a duke, hence Monaco is a koukoku).
In Gundam, however, Degwin is NOT a duke, but a "kouou" (公王). This title is completely made up. The 公/kou in 公国/koukoku comes from this made-up word, and thus it is NOT a duchy/principality. Since there is no real translation for kouou, there is thus also no real translation for koukoku.
The "ou" (王) in kouou (公王), however, means "king", which makes it clear that yes, he is indeed royalty. So there's nothing wrong with referring to Minerva as a princess.
Also, there is a boss fight in Dark Souls with "the four kouou" (四人の公王), which is translated to The Four Kings in the English version, so there.