True, Grass is the worst defensive type next to Ice, and offensively is quite lackluster. I'm not denying that Grass types can be good. Ferrothorn, Torterra and Celebi are all great pokemon. But Grass types are definitely disadvantaged because of the type chart.
As for the worst type, I'd say either Poison or Ice is worse. Poison is terrible offensively, though pretty good defensively. Ice is very good offensively, hitting 4 key types including Dragon, but has no resistances bar itself and crippling Rock and Fighting weaknesses. The Ice type is actually renowned for its types attacks rather than the pokemon themselves.
As for the worst type, I'd say either Poison or Ice is worse. Poison is terrible offensively, though pretty good defensively. Ice is very good offensively, hitting 4 key types including Dragon, but has no resistances bar itself and crippling Rock and Fighting weaknesses. The Ice type is actually renowned for its types attacks rather than the pokemon themselves.