How much does the average driver have to understand about automotive and mechanical engineering before they can form a reasonable criticism about defective/fatal design?

Physical products are much easier to intuit your way around than software. For cars, effectively everyone knows what the engine, fuel tank, tires, and drive selection lever are (though they may call the last one a "PRNDL"). Even if they don't know exactly how an alternator works, they understand that they need to drive their car around after they jump it to recharge the battery.

By comparison, I think most people don't even know the basic architecture of a computer, let alone the applications that run on it. Most people don't have an understanding of the network stack - I consider people ahead of the curve if they know the difference between LAN and the wider internet. Some people even think it's possible to download more RAM...

Driving follows physics mechanics, is a skill and is much more involved than search. Search is designed to be a black box for most people.

But ignoring that, no, I still would not like to see people professionally criticizing processes and products that they do not understand. Some technical knowledge is a must for people who do professional talks or presentations, write books and research papers and teach on the topic.

For algorithms and ML, I would expect at the very least a bachelors in math/CS or the equivalent in work/personal project experience.

criticism follows understanding, so the only honest answer is “quite a lot”

i realize this isn’t widely appreciated or accepted in the age of critical theory, when social desirability leads people to complain about everything in the guise of “fighting oppression,” but it’s a simple real-world fact that no one solves problems from a position of ignorance