The Smoother the better it will work.
Can I apply the clear product on top of a traditional sligthly textured wall or does it have to be smooth ?
If you think you could write on the wall right now without a problem then it should be ok. If not then you may need to lightly sand it. If it is too textured it will not only be hard to right smoothly but it will be harder to erase due to ink getting trapped in little nooks and crannies.