Now if you were a scientist who earns a living from science, then being called scientifically stupid might be quite an insult. But I don't think you are, right? So what difference does it make in your life to be "scientifically stupid"? That's what technology is for, dear: packaging science into scientifically-stupid-people-friendly boxes for blurheads like you and me. We don't need to know squat about semiconductors to use the computer, or electron guns to use the television, or hydrocarbon combustion and differential gears to use a car.
Now, I'm not saying that I think you're scientifically stupid. I think I'm scientifically stupid (or at least not where I want to be yet). But why does it sting so much? It isn't that bad, right? What has happened is that in the Western culture centered around science and technology science has become synonymous with intellectuality. In other words when someone calls you "scientifically stupid" what you really hear is: "you're stupid, fullstop." As I've shown, that really doesn't make much sense, does it? But we've encouraged it by all-but-deifying science (and YECism is a part of it, too - trying to make the Christian faith credible by making it scientifically credible, indirectly bowing down to the powerful image of science). Someone smart is an "Einstein" or a "rocket scientist", a genius is someone who invents some new technology. What would you feel if I said "You're as smart as Picasso!", that wouldn't feel like a compliment at all, would it?
Being scientifically stupid isn't all that bad ...
Secondly, most TEs here actually don't assume outright and immediately that YECs are scientifically stupid. Proof? We bother to try to reason you out of it. By directing you to TO, say, we assume that you understand the methodology of TO and the terms it uses. If I try to show you that radiodecay rates are constant, it means that I assume you know what radioactive decay is, how it works, what an atom is (which is actually brilliant: people weren't even sure that atoms existed two centuries ago). If we really thought you were scientifically stupid, we wouldn't even bother. Of course, there are a few people here who will do just that with a PRATT, swipe it off without giving it a second thought, but by and large most of us are willing to explain the science behind our conclusions. You may be scientifically stubborn in refusing to accept our explanations, but we don't consider you scientifically stupid.
On the other hand, how do you think we feel every time YECs come along and effectively say that we have no right to be Christians simply because we believe in evolution?