That's not hateful, it is quite thoughtless and inconsiderate to put it in the book assuming there was no prior knowledge but it's not hateful. He actually goes on to say that he enjoyed trying to make her smile or laugh because she didn't laugh often.
To say it's hateful that he did it at the time is laughable - he's talking about being a 12/13 year old boy. Lord help us all if the plan is to start judging us on things we did or said when we were 12
.