I don't know about that, I mean they aren't polite to Donna, and I think they would treat her like family... not some title. Can you honestly see Yuna asking people to call her that?
Donna is a cow
And she doesn't have a Dad for High Summoner.
Cow or not, if your planning to sacrifice your life to give the planet a holiday, you deserve respect.
I mean sure she quits n everything but they didn't know that at the time.