Roofing felt makes your roof/terrace waterproof by stopping water from draining away to anywhere below and relies on evaporation for water to ‘disappear’ from the surface.
Artificial grass allows liquids to drain away from the surface to whatever is below it, so it is usually only installed on top of something with good drainage capabilities.
These are two opposing functions!
If you cover your roofing felt with artificial grass it will make it much more difficult for the water to evaporate so you may end up with water sandwiched between the two surfaces. In colder periods where it is less likely for water to evaporate easily there is a greater chance of this water becoming stagnant.
If your roof/terrace is sheltered and does not receive much rain this may not be a problem.