It's not offensive to think that of her if you actually understand that there is a clear and distinct culture shift depending on where you live and in what community.
I live in a fairly conservative urban environment. Around me, the term "retarded" is used freely to describe mental retardation, and has no negative connotations unless it is clearly being used as an insult (and even then, you're more likely to get cries to stop misusing the term because it's offensive to people who are actually retarded than anything else). Usage of the "politically correct" terminology is almost universally met with derision and eye-rolling.
But I have family that lives in a rural environment far closer to what the Pioneer Woman lives in. And visiting them is an eye-opener to this sort of thing. The usage of PC terms would never even come up. It just doesn't happen. No one would blink an eye if you used them, but they wouldn't realize why you were doing it, or that there was anything wrong with using the terms that most of them had been using their entire life. Change of that sort comes far, far slower than it does to urban environments, and anyone who has been a part of that culture, even for a short time, can realize that.
Until she stepped into the culture outside her hometown (with things like the writing deals and the Food Network deals), her only likely exposure to a culture where the term "retarded" was considered inappropriate was on the internet. Which, unless she was extremely active on message boards, would mean that she wouldn't encounter the PC terms except in comments and complaints about her own works. It's not a common discussion topic, since most of the internet doesn't actually focus on culture trends, if you're not actively looking for it, which she had no reason to do.
You say you have to be living under a rock to miss the clear negative connotation that "retarded" has in today's culture. But "today's culture" is not a homogenous one, and it's more offensive that you think it is, than it is that I acknowledge the differences.