I’m a teacher. I have mixed views on this.
On the one hand, your daughter is doodling in her exercise books regularly. That will be making her books look an absolute mess, and her teacher will (unfairly) be held responsible if a member of SLT or Ofsted observe the class and pick up her book. It just doesn’t give the impression of pride in her work. I mean, if she ever gets a job where paperwork is required, is she planning to doodle all over that? Can she prevent herself when it’s really needed? Because it’s really needed in her school books.
On the other hand, I am really not a fan of punishments or behaviour solutions which have nothing to do with the reality of what’s been done, and bear no relation to what would happen in real life. 5% off an exam score is arbitrary, decided by the teacher, and unrelated to the doodle. Your daughter’s learning and performance in the exam got 96%.
Would the explanation at the top of my post change your daughter’s behaviour at all? Would having a piece of scrap paper to doodle on in lessons (maybe even after exams, although that wouldn’t be allowed at GCSE) stop her from drawing on her work, or would she still draw on her work? Can she be made to understand that drawing on your work is a bad idea, so that she’s motivated from within not to do it any more?
If she can’t, then maybe some kind of punishment-consequence is necessary, but again - my ideal teacher would relate it to the thing done. Stay behind at break time to erase/go over with white paint/stick cutout pieces of paper over every doodle to restore your book to its former glory, for example. Not a random 5% off an earned score.
However, with all that said, for your daughter’s sake, please don’t complain to the school about this or defend your daughter in any way. The teacher does and should have a choice to respond to this. I don’t think it’s the best response cognitively, but it’s a response that says ‘I won’t let this pass’, and that’s better than tolerance of something established as wrong.
If you go challenging the teacher over this you really will be ‘that parent’. The exam score isn’t going anywhere official. Instead, back up the point at home by reinforcing the real reasons why your DD should stop the doodling. Aim to find an alternative way of occupying her hands that’s acceptable at school and can pass the time. And when the time is right, find ways to encourage proper drawing and painting at home.,