Even Ramos still makes mistakes. And he's 33. Nobody is perfect. There is no PERFECT defender. People need to understand that.
De Ligt is the best ever defender for his age. Like Lippi said, even Maldini and Nesta weren't this good at 19
And i think he's already better than some high (or semi high
) rated defenders like Maguire and Stones... who are much older (and probably more expansive).
De Ligt has played over 100 games at Ajax. Played a EL final, Nation League final, CL Semi-final, 20 caps already for the national team. He's the captain of Ajax. Pretty impressive for a 19 years old kid.
At the moment i think The Ligt belongs to the top 5 best defenders. Maybe top 3 after a good season at Juve/Barca/ManUnited (if he don't get injured).
But nobody can look into the future. Nobody knows how good he will be when he's 25.