Hi, when I studied English for exam to enter the high school, I was taught that the reverse of "be good at" was "be poor at". Do native speakers of English say like this?
That is correct.
For example, "John is good in Science." or "Judy is poor in English".
In this context, the word "poor" does not refer to material wealth. It means to display a lack of proficiency in English.
I tend to agree with Susanne. I remember my primary school teacher using this adjective when she wrote on my report card. I seldom hear it anymore though the meaning remains.
Hope this helps.