I view it as a bit of a vicious cycle. I think most of us here agree that general math education in North America leaves something to be desired, with much of the gap originating from poor foundational math skills, which are closely related to the effectiveness of math instruction in elementary school--which leads back to the deficiencies in math skills (let's leave aside descriptors of intelligence, and speak strictly of skills) of many elementary school teachers.
I have in my circle quite a few teachers, at all levels (as you might expect), some of whom are really quite good at pedagogy, and also very intelligent in their areas of strength. Those with both typically are able to acquire content knowledge when properly instructed, often for the first time as adults. For example, I know special education teachers who learned 10th grade math while teaching it alongside math content specialists in an inclusive setting, and now have the skills to pass the high school math teacher certification tests. An elementary teacher I know found teaching math in a curriculum aligned to the much-maligned Common Core frameworks to be eye-opening, as, for the first time in their experience, reasoning, problem-solving, and multiple methods were included in math instruction. (I will note that this teacher received training in the new curriculum, and actively engaged with it, unlike many of the others in the district.)
So yes, quite a lot of certified teachers lack foundational math skills, but a fair amount of that can likely be ascribed to, well, the lack of good math instruction in K-12 education.
