Regardless of the subject matter writing a Mr Men book is ridiculously simple for a 15 year old. That said this project could yield some dubious results. Definitely not helpful. The point of Mr Men is to be light hearted so you would end up with sonething really messed up. See below.
^Mr Moustache was very particular about his moustache. In fact, he was very particular about a lot of things.
He lived in a far away place called Germanyville. Lots of different kinds of people lived in Germanyville including Germans but Mr Moustanche, who was very particular thought that this was very peculiar. And you know why don't you?
He thought that only ethnic Germans should live in Germanyville, it was called Garmanyville after all. But all the other people liked it in Germanville and didn't want to leave.
One day he met Mr. Jew at the super market. Mr Jew wasn't a German. 'Why hello,' said Mr Jew and pulled on Mr. Moustache's moustache 'that's a funny moustache you have there'. Mr Moustache went bright red from head to toe because he was so angry. And you know what he decided to do next don't you?
He decided to kill Mr Jew and everyone else he didn't like for similarly ridiculous reasons.^
I would definitely not be pleased if my children were set this task, even if they were younger where the task would at least he appropriate to their skill levels it would send completely the wrong message.