Please sit down, take a deep breath and try to re-read your own post as if you're reading a stranger's post.
"I do not pressure her..."
Yet she might "continue to trend toward mediocrity...."
How is a single B on a report card mediocre?
How is this in any way a "trend"? This is her first report card. One report card does not make a trend or even start one.
You don't see it but you are pressuring her, or you will be soon. She likely has already picked up on your focus on grades even if you have not said a single word to her about them. And...how do you know what the grades are "likely" to be? The card has not come home yet? Why are you concerned with this before you even see the report card itself? Why not praise her for all As and one B rather than preparing your speech to her about why she must do better?
You do "come across as an over the top mom who will not allow anything less than an A" -- as you yourself put it. Please, please ramp back the grade expectations. Put the expectations instead on whether she is learning anything; whether she is enjoying the learning process; whether she retains what's important and makes good connections between what she learns and her everyday life.
I am writing this as a mom with very high academic expectations for my daughter (11, in 6th grade) and please, take it from my experience, your expectations won't matter when you eventually hit the art teacher who gives your child only B's because she is just not as good at visual art, or the music teacher who reveals to you that your child is not as musical as you thought, or the math teacher who gives a C on a few tests because your child got the wrong idea about how to do certain types of problems, and yes, your child will get a wrong idea eventually....This is learning. Yes, hold your child to a solid standard and expect good grades but not perfect ones, or school becomes a stress and a chore.
Especially at her age, look more at her marks for effort and her behavior marks than at her academic grades. If she gets all As but gets comments that she does not cooperate well with others, for instance, you should be concerned, not focused on just the wonderful As.
Our system just this fall dropped letter grades for a number system with much more descriptive report cards that indicate whether a child grasped all the concepts taught that quarter, etc. We'll see how it goes; it's an adjustment for us both; and she will be right back to letter grades next year in middle school. But it does force us to rethink things and focus on what she's learning and not on the grades.
By the way: "After all, the world is not getting any less competitive" -- Sounds like parents in our area who, from preschool on, are angling for the path to get their kids to the right elementary school so they can be in the right middle school so they can take a ton of tutoring classes so they can pass tests to go to the intense math/science specialist high school -- whether the kids want it or not. Please, think about whether you are already on the path to pressuring your child to be perfect and competitive. I see so many stressed kids of all ages here. I expect a great deal of my kid, who is in the gifted program, does Science Olympiad, etc., but the emphasis is on participating and learning something -- not winning, winning, winning. You may be thinking it's just about a B now, but if a B now bothers you this much -- you will be on the road to lining her up for Ivy League entry by the time she's finishing elementary school. Seriously, I see it here. Please don't make that mistake, however well-meaning it is.