Thinking that Sarah is stupid is not the simplest explanation.
Couple of reasons:
1) Sarah got into university
2) Sarah finished university
3) Sarah pass approximately 32 course at university
4) Sarah maintained a high enough average to be select to work in the corporation she is in now.
5) Sarah was chosen as the expert in her field
There is only two things that point to Sarah being stupid:
1) Copying others' work and thinking that a simple change of font is sufficient enough to mask it.
2) What your mother said.
As you would have noted, evidence for Sarah being smart can't simply be explained away. It would require a few professors and classmate helping her out. It would require someone botching the interview and the selection of an expert. This isn't the simplest explanation.
If we assume Sarah is smart, why did she make such a glaring mistake and why does your mom said what she said? A possible reason why Sarah did what she did was because she didn't want to do it again. I assume the effort that Sarah placed into copying and the conversation with Michelle was not more than 30mins.
People who do a good job are normally rewarded with more of the same work. So, instead of 30mins of work, Sarah might have ended up putting 300hours of work into something that will most certainly not yield a high payoff for her. So, in this case it might be best if she pretended that she is stupid, which would basically fast track her career by months if she was to working on stuff with high payoff instead.
But, you could tell Sarah's boss about her incompetence. No you wouldn't; the easiest thing is to ask for another expert. Even if you did, she can simply say she was going to write legal a letter that it might be in the best interest of the company to purchase the copyrights of these paper, instead of having a highly qualified person wasting time and money on something that might essentially be fruitless.
Sometimes it might not be in your best interest to appearing competent all the time. Appearing competent all the time is akin to a greedy algorithm which might not be the optimal solution to achieving your end goal.
In terms of the author's mom, maybe she is right but it doesn't apply here.
In summary, the simplest explanation is that Sarah is smart, but for whatever reason she chose to appear incompetent in this scenario; I have outline such a scenario above. If Sarah is stupid it would require, more than likely, over a dozen people willfully assisting Sarah's incompetence, but if she is smart it just requires her to play dumb for 30mins.
This is not to prove comprehensively that Sarah is smart but really to show that other simpler explanations exist. If I was Michelle, a lot of alarms will be going off in my mind, and I would not simply dismiss it as she is stupid.