There's a lot of talk in this novel about the ideal roles of men and women. Men are supposed to be strong, brave, and decisive, and women are supposed to be sweet, pure, and innocent. Of course, those roles get mixed up on occasion (as hard and fast gender roles tend to do). Sorting through what Dracula is really suggesting about ideals of masculinity and femininity is part of the fun of reading this novel.
Despite Mina's maternal femininity, her "man's brain" and Jonathan's passivity serve to balance the power dynamic between them.
Mina represents a late Victorian ideal of femininity: she is intelligent, capable, and willing to enter the workplace, but only as an assistant to her husband.