Do remember that all of the main mamas in the game have a single common trope; sacrificial lamb. The author/developer of the game made the mamas as a way to have their respective characters have character development. Mamania was to be a sex slave of the bandits so that her daughter can have courage, The teacher was to die protecting her students so that the class will have the will to always fight on and protect one another, Mama Terria was to be sacrificed to the world tree so that her daughter will be free, and now I'm thinking the Queen already knows that the prince is going to assassinate her for the crown and is okay with that.
The Queen is okay with dying, but I'm not sure what her end goal is. Despite sacrificing herself, I don't think in the original story the prince will be redeemed and will be a good ruler due to her death.