More to the point, in my experience the kind of people who actually do great things are not the kind of people who spend their time thinking about how mediocre, unambitious and lackluster other people are.
It's not necessarily dismissing them. Face it, some people are mediocre and unambitious. It doesn't say anything about what kind of people they are, but it says something about how they are intellectually and how pleasant working with them will be.
More to the point, people who are unambitious are mostly harmless. It's their right to be unambitious. It's those who start trying to persuade ambitious people that their ambitions will fail that are annoying.
Ok, I dare you to go up to someone and say, "You're medicore and unambitious -- not that there's anything wrong with that! It just means you're not as intellectual as me and I probably wouldn't enjoy working with you."
You can judge by their reaction whether they think you're saying something about what kind of person they are.
I still maintain it says more about you than them, though.
Everyone has a story. Dismissing people by calling them mediocre, unambitious, or lackluster says more about you than them.