How can I change the text “Auto added by WPeMatico” in new categories?

You can replace the text with a custom filter in your functions.php file, such as the example below:

add_filter('wpematico_addcat_description', "My_function_name");
function My_function_name($catdescription, $catname){
	$catdescription= "My NEW COOL DESCRIPTION";
	return $catdescription;
}