The first parameter should be get (or not?) through the following statement in the where clause:
source.asExpr() = call.getArgument(0)

There is a big difference between a parameter and an argument.
source.asParameter() = ...

Thanks for the hint.
I tried with:
source.asParameter() = call.getMethod().getParameter(0)
The result is different but I only got 2 results when I was supposed to get 6.

In addition, i don't know why the parameter "Object value" isn't get as a result, although it is the first parameter of a "isValid" method inside a class which implements "ConstraintValidator<>"

For the future, please post your current query as text :)
I suggest using Quick Evaluation on both parts of the and and also on the whole.

method.getName() = "isValid" and
method.getDeclaringType().getAnANcestor().getASupertype().getName() = " ConstraintValidator<>"

You're trying to find the overrides of a method, but I think that this will not work this way.
There is a overrides predicate that you can use.

After some time, you will probably come to the conclusion that overrides does not work for our use-case.
This is due to generics.
if so, have a look at github/codeql#3490.

And as always, the greatest tool in debugging is Quick Evaluation.

I did some tests using Quick Evaluation tool as you suggested.
I figured out that the problem was due to the method access call, so I tried with the following query:

predicate isSource(DataFlow::Node source) {
  exists( Method method |
    source.asParameter() = method.getParameter(0) and
    method.getName() = "isValid" and
    method.getDeclaringType().getAnAncestor().getASupertype().getName()
    = "ConstraintValidator<>" and
    )
}

This time the result seems almost the right one.
I got 8 results because there are 2 isValid method not overridden too, so I had a look to the link you shared.
I tried with a new query:

predicate isSource(DataFlow::Node source) {
  exists( Method method, Method overridden |
    source.asParameter() = method.getParameter(0) and
    method.getName() = "isValid" and
    method.getDeclaringType().getAnAncestor().getASupertype().getName()
    = "ConstraintValidator<>" and
    overridden.getSourceDeclaration() = method and
    method.overrides*(overridden) 
    )
}

However, the result isn't change (the same 8 results as before), but i think that the use of the overrides* method should be right.

One big problem is this:

method.getName() = "isValid" and 
method.getDeclaringType().getAnAncestor().getASupertype().getName()
= "ConstraintValidator<>"

This will select all method that have the name isValid.
What you are trying to do here is selecting only methods that are derived from ConstraintValidator.
But consider these (pseudo-code-ish) classes:

class Foo implements Bar {
public void isValid(String clearly, String not, String a, String constraintValidatorMethod);
}

class Bar implements ConstraintValidator {}

Your query would flag the above isValid method because it is declared in Foo. And Bar is an ancestor of Foo, but also ConstraintValidator is an ancestor of Foo.

Instead I'd suggest using something like this:

isValidMethod.hasName("isValid") and
  isValidMethod.getDeclaringType().getName() = "ConstraintValidator"

this will get you all isValid method that are defined in ConstraintValidator.
This will give you two results. I don't know why there are two results, but it is probably related to Generics...
Then I'd use m.overridenOrInstantiates(isValidMethod) to find all methods m that override the isValid method from ConstraintValidator.

Finally I managed to resolve the Step.
I used your suggestion, but keeping the getAncestor method.

predicate isSource(DataFlow::Node source) {
  exists( Method method, Method m |
    source.asParameter() = m.getParameter(0) and
    method.getName() = "isValid" and
    method.getDeclaringType().getAnAncestor().getASupertype().getName()
    = "ConstraintValidator<>" and
    m.overridesOrInstantiates(method)
    )
}

I used the method m in the getParameter function instead of isValidMethod and used your override instruction.
The query gets 6 isValidMethod overridden.
Thanks a lot :)