Wait a minute! Disney made use of the ‘evil queen’ thing less common than you think!

(How I reconciled this in my AU)
 
Fausticorn is Queen, Alicorn representing Craig is King.
 
The stinger?
 
They’ve been missing since before recorded history, hence Discord’s “usurption” as well as Celestia and Luna’s ascension.

Technically, “Prince/Princess” doesn’t always mean “child and heir of the King”. Principalities are basically monarchies where the head of state is called a prince, and in many cases they were just as powerful and influential as any king.  
Of course, since many children and adults didn’t know what a duke is, which was Blueblood’s original title, I’m not surprised that most people don’t know about principalities.

Rapunzel’s ma was pretty chill though.

Emperor is my favorite title. Is has a weight and majesty to it that no other human title shares.

Hail Celestia! God save the queen!

The pilot says “the two sisters maintained balance for their kingdom and their subjects, all the different types of ponies”, so I don’t think it was a goof.

Other than Disney mucking things up, there are different reasons for giving Celestia the title of princess instead of Queen.
 
There is the different weight of titles. Though because I’m not well versed in real life nobility systems and wikipedia has so far been unhelpful to this venture, I haven’t found too much difference between titles other than ranking and what the territory they rule over is called.
 
In setting, the most likely answer would be the fact that Celestia and Luna share power over Equestria, as one of the Background Ponies here noted.  
Or it could have some mirroring with real life. There might be some bad connotations around the title of Queen left over from the Unicorn nation before the founding of Equestria. Though said nation’s power structure might be still around if Flim’s (or was it Flam? I can never remember which is which) comment about the “Kingdom of Canterlot” is taken as a bit of world building instead of a writer’s goof, which raises even more questions about their title system.

What if we say that since there can be only one queen. Celestia, to share power with her sister Luna, had to choose another title that they could share.

Empresses are actually for some reason in the same category as princess, usually. They will be regent of x peaceful kingdom under attack by an evil king/general or be cutely ineffective and therefore prime targets for a military revolt. Well, not Disney as I haven’t seen them use Empress, but in general that’s how I see empress get treated. I think that comes from China mostly.

What makes Empresses?  
Stereotypes like this is why I somewhat dislike Disney. If anything Kings and Queens tend to be more wiser in real life than the Princes and Princesses are.  
In many anime, in Kingdoms it is the Princes who plot or anticipate the death of their King so they could take power for their greedy desires.  
Disney rarely touches the “evil empire” cliche though.  
In Empires it is usually the Good prince or princess who join the resistance to fight against the evil Emperor.  
Yeah I don’t get the logic here.

Hail Queen Molestia!

For Celestia? I totally would.

@Fenwolf2003
 
Bow for your Queen slave!!

Queen Celestia sounds like a Dominatrix.

@ElusiveDelight
 
Maybe we can agree to Empress Celestia?

@Prof.NightJack
 
Well Disney has gotten better it seems.
 
But yes they pretty much single handedly ruined any notion of Queens being good. In fact the only way I can think of to remedy that is for them to make a movie that has a good Queen.

Honestly, “Princess Celestia” sounds better then “Queen Celestia” anyway.

@Niggoslav_Krawczyk  
I would pay ALL my money to see something like that!

That is so derp-o, though. I think we need an ep where they go to a foreign land with a queen and a vain bitch daughter, and the vain bitch daughter never takes the title because ‘that would imply I dethroned my mother for her title, when I’ve ALWAYS been better than her!”

Well, “Holy Empress of the Sun, May She Live Forever in Her Castle-Atop-the-Mountain” would be too much of a mouthful.

@boogstersu2 @Yorec  
Ya… that’s why I don’t really like Disney… most times.

She is a Queen in all but name though. It’s just that because of Disney the word Queen isn’t marketable to little girls anymore. Since you know they did the idiotic by making every single Queen a vain, megalomaniac bitch.

“Queens are evil. Princesses are nicer.”  
~Disney logic