"Princess" is also the huge sow in Elwynn. It all depends on the context. Generally there's only one princess in the dungeon, so it's easier to refer to her by that title as it's a recognizable position, than to remember names (I know people who can't read LotR because of all the names).
And since the particular boss is always evoked in the context of that particular instance (e.g. LF DPS for Mara princess), not mentioning the name doesn't really matter.
On the other hand, you have memetic badasses nobody refers by title, but by name. Like Hogger is widely known as Hogger, not as "that gnoll boss" while let's say Deathclasp is usually referred to by its name only after the person(s) needing help read the quest title.
In raids, same for Ragnaros (the Firelord), Nefarian, nobody calls him lord (which is rude) or Onyxia, who isn't addressed as broodmother either.
And in the last tier there are bosses nobody cares about or remembers so BFD's Ghamoo-Ra is just called "a turtle".
Why is Majordomo domo, I don't know. I think it's because most of people on the internet suffer brain damage and can't process names longer than two syllables.
Personally, when you say domo, I imagine this guy: https://linxspire.files.wordpress.com/2 ... mo-art.jpg
Be glad you weren't on Czech TBC servers. The nicknames there sometimes grated against all sensibility. Magtheridon was called Magnet.
And (regardless of language of the server), nobody ever knows how many Us and Ls Gruul the Dragonkiller has in his name, leading to him being mistaken for Grull, a questgiver in tauren starting zone.