I think a “Queen City” is a city that is not the capital city of a state but has the biggest population. I know Burlington VT and others embrace this distinction more than others.
Cincinnati's "Queen City" sobriquet derives from an 1857 Henry Wadsworth Longfellow poem titled Ode to Catawba Wine, in part:
"And this song of the Vine,
This greeting of mine,
The winds and the birds shall deliver
To the Queen of the west,
In her garlands dressed,
On the banks of the Beautiful River."