Should I check the input variables (pointers) in a mock function?

I am writing a series of mock functions in C using CMocka

Some of these take pointers as input variables and I am not sure whether I should check them (ptr != NULL) or not.

In general, is the mock function responsible to perform input checking?

If yes, how should it behave when an error is found? Should it use the assert functions provided by the framework?

If your mentioned pointers are parameters which are passed to some mocked functions you can check them with check_expected(...) and expect_value().

void function_under_test(){
   char c = 'c'; 
   int ret;
   //...
   ret = subfunction(&c);
   if(ret == 0)
      printf("Success");
   //...
}

int __wrap_mocked_subfunction(int* p_paramater){
   check_expected(p_paramater);
   return mock();
}

test(void **state){
    expect_not_value(__wrap_mocked_functions, p_paramater, NULL);
    will_return(0);

    function_under_test();
}

Errors are then reported automatically.

An example can be found here: https://lwn.net/Articles/558106/

If you really need to check them depends on you and your code and your opinion and your requirements.