Is it mandatory to use AEC qualified component in automotive applications?
What is AEC ?

From here:

The Automotive Electronics Council (AEC) is an organisation based in the United States that sets qualification standards for the supply of components in the automotive electronics industry.

So the AEC only sets the standards.

Let's imagine a silly example:

AEC standard 12345A specifies that capacitors need to have a colour.

So does this mean that the AEC will sue you if you use Black capacitors (assuming that black is not a colour) in your product for the Automotive market ?

No because the AEC only writes that document, it does not check (or even care) that you use black capacitors.

Only if you are going to sell your product to a car manufacturer that manufacturer might require that your product meets AEC standard 12345A.

The nice thing about having an organisation setting the standards is that all car manufacturers can use that same AEC standard 12345A  if they want that.

Imagine all car manufacturers having their own standards, you'd have to read them all (provided if you have access to them) and meet all of them. That's a lot of work !

It's much easier when you can say: My product meets AEC standard 12345A

Then everyone knows what we're talking about.