I think it's wildly inappropriate - either the 11 year old knows what the cable ties are for, in which case he shouldn't be taking them, or he has no idea - in which case, he shouldn't be taking them.
The mother says he hasn't read the book, so it's not even any kind of 'celebration of literature/free speech/ideas' - and nothing to do with world book day.
I'm sure his tongue was in his cheek, but I don't really see that that matters tbh.
I sympathise with the parents of Elsas, because I always found that most characters we could think of didn't really wear anything distinctive - just as most people don't! - so I can see why you'd go for a costume that's identifiable over one that's literary, I suppose. This is the problem with fancy dress! (Although I must admit I liked doing WBD costumes, and rather miss them!).