In gen1, it used to be the second worst type in the game, just ahead of Bug. Sludge was the most powerful Poison attack with 65 power and only 2 Pokemon learned it. Being weak to Ground and Psychic didn't help, and their best status move was glitched and didn't even work when your opponent switched.
Nowadays, Poison isn't actually all that bad. With the introduction of the Fairy type and heavy use of stall tactics with a Toxic attack that actually works, it gets the job done.
I think it was better last gen, though. Psychic types seem to be seeing an increase in use thanks to Tapi Lele.