Artificial planting is perfectly fine for a QT, they will help de-stress new fish and help them feel safe and secure whilst you make sure they are free from any issues just as living plants would. Tbh using artificial in a QT is better in a way since after each use of the QT you can rinse the plants off in old aquarium water and keep them ready for use the next time without them becoming stagnant in the unused aquarium...as you rightly say, it is a QT, so not in full time use. You can empty the QT, rinse the artificial plants and put them away til you need to cycle again for new or ill fish later.