The meaning of the sentence is shown through the double negative.... "You can't pretend he's not careless..." Here means- It's not possible for you think in another way....even if you were pretending you still wouldn't be able to believe something different. It emphasizes the point that Hagrid is so careless that even if you were pretending/dreaming/creating an alternative story he would still remain careless. It still wouldn't be possible for you to say he isn't careless. This is a pretty fitting statement for Hagrid. He does many careless things throughout all the books so it is probably why the author really makes a strong statement. Hope that makes sense. I tried to give a little more information so that you would understand the language and style better. Let me know if you have any other questions. Enjoy the book! I loved them all.

double negative makes a positive.