Whether taking driving lessons or a practical driving test, if you feel that adjusting your mirrors prior or during the parallel parking manoeuvre will improve your ability to complete the manoeuvre safely, then it’s perfectly acceptable to adjust them accordingly.
You may find that adjusting a side mirror so that you have a better view of the kerb is beneficial as it may help you by preventing you from hitting it, worse still, from mounting the pavement.
It’s best if you adjust the side mirror before the manoeuvre begins, but if you do need to make adjustments during the parallel parking manoeuvre, ensure you stop and secure the vehicle before doing so.