In Brawl, due to type effectiveness, grass attacks deal 110% knockback to Squirtle and 90% knockback to Charizard. The only playable characters that can use grass attacks are Ivysaur and Kirby (by copying Ivysaur).
Any benefits of this effect were removed in Super Smash Bros. Ultimate with the removal of type effectiveness.
The Subspace Emissary
In The Subspace Emissary, almost all enemies have the same resistance to both slash and grass attacks; Primids are the only exception, taking 1.3× damage from slash attacks but 1.0× from grass attacks.
List of grass attacks by characters