I have no information, only speculation.
I give several completely arbitrary personal names to members of the family.
Possible Prince Charming's full name and title is Robert Fortuna, Prince Charming. That is, a man named Robert Fortuna, with the title of Prince Charming.
Or perhaps the Fairy Godmother's maiden name was Mary Fortuna, hereditary Dama (lady of) Fortuna, and she married Albert Charming, Prince (of) Charming, and became Princess of Charming. When her husband Prince Albert deed she became the dowager Princess of Charming, and her son Robert became the Prince of Charming.
So then the full title of Mary Fortuna Charming would be Dama Fortuna and dowager Princess of Charming.
And maybe in the world of Shrek 2 it isn't customary to refer to widowed noblewomen by dowager titles, so the Fairy Godmother was only called Dama Fortuna, her hereditary title, and not by the title of her deceased husband.
But that doesn't explain why Fiona would daydream about being "Mrs. Fiona Charming" instead of "Fiona, Princess Charming" or "Princess Fiona of Charming". It is possible that in the world of Shrek 2 not all wives of princes get the title of princess.
Perhaps wives of low rank in the world of Shrek 2 have to be content with "morganatic" or unequal marriages and do not get their husband's title but might possibly be granted some lesser title for themselves and their children. That was the case among European royalty and nobility from about 1450 to the present, through the rule has been relaxed in the last few decades.
Of course Fiona was the daughter of the King and Queen of Far, Far, Away, but she was also sometimes in the form of an ogre, and might have believed that would make her considered a member of a lower species and thus not eligible for an equal marriage with Prince Charming.
So maybe Fiona daydreamed of becoming the wife of Prince Charming without expecting that she would automatically be made his princess