I've seen people try to make a poisoner, but it's usually a side gig.
First, PCs are usually meant to take on enemies in one go and the game rewards you for defeating them quickly, so poison - which requires time to ramp up - is generally not seen as a good option. While you're busy poisoning the enemy, the barbarian can just cleave them in two - which takes them out of the game a lot faster. Second, it costs money to make or buy (in most cases), which eats into the party's gear budget, unlike magic such as cloudkill or the poison spell.
I'd probably suggest the alchemist, either the core/vivisectionist for a capable utility guy/gal with some poison knowledge, or the eldritch poisoner or toxicant archetypes for a poison specialist. The Toxin Codexer investigator is also fairly good imo, not as strong as the alchemist in a fight but better in social or investigative situations. However, in either case I'd treat poisons as a backup option, like maybe lobbing a grenade at that reedy git in the back that talks funny, rather than your main combat contribution.