I've always considered ambition as something for those who can't enjoy the present. They're always so worried about the future that they can't see what's just infront of them. The now. It's always about the future, it's always about the straight A's and it's always about that "someday."
Someday they'll be rich and have a good job. Or something. I don't know. I just know that all those tomorrows until Someday will become yesterdays. A frightening alot of yesterdays without any real significance or importance.
Not saying that having some ambition is a bad thing. Just as long as they don't overwhelm you.
It's not that bad being mediocre.