Almost every detail of Pokémon from 3rd generation games remains unchanged when they are brought all the way up to Sun and Moon, including the Pokémon's nature and ability. There are a handful of exceptions (Gengar, Scolipede, Chandelure) but you will still have a careful Nidoran (F) with Poison Point. Depending on its current moveset, PP of moves may have gone up or down.
Source: experience. I still have the Venusaur from my first playthrough of LeafGreen, still Impish and still Overgrow.