This unpleasantness is your awakening to the reality that kids don't always do or say nice things. They're preschoolers, they're learning, their finding a balance (way too slowly, I know) between dependence on adults and a small amount of autonomy. In fact, I'd suggest you be very careful about labels like "Miss Bossypants" and so on. That doesn't help, and your attitude will be picked up, no matter how subtle or guarded you think you are, by your daughter.
What you do is strengthen your daughter. In age-appropriate ways, you let her know that other people's acceptance does not define her. Whether it's a 5 year old, a 10 y.o., a boyfriend, a teacher, a boss, even a stranger in a store or on a subway....there will always be people who are blunt, thoughtless, mean or horrible. You will be doing this, in some fashion for 15-20 years. Maybe longer. What this kid is doing is not what determines your daughter's view of herself.
You can mention it to the teacher by asking how they recommend you deal with these situations - when to intervene, when to let it go, etc. We cannot protect our kids from hurt. Nor should we. We have to give them the tools to deal with this sort of thing, but I can assure you that you will be absolutely miserable if you let every slight set you off.
Ans while your description of your daughter is what I would expect - we all see our children's gloriously good points - it's really good to admit that your child is not, and will not always be, sweet and delightful and thoughtful. In fact, you don't know what she may already have said to these other kids. Stay open to all possibilities, and start learning the very hard parenting lesson that we can't buffer our children from everything.