jmartkdr2
Hero
While there is nothing wrong with it per se, there are other cantrips that mechanically are better IMO.
As others have said, Poison is a common resistance (along with Fire) and quite a few do have immunity. It compensates for that with the d12 a bit, but also relies on a CON save, which CON is one of the highest ability scores overall in the game. Since creatures with high HP will likely have good CON scores, that d12 damage counts for even less.
But, all that being said, if you want to take it, go for it! It will still prove useful enough and you don't always have to take the "optimal" choice to have fun.
Specifically, all but one creature in the undead category are immune to poison - so if your dm like zombies (or just dislikes using intelligent enemies all the time because you always end up talking and the fighters getting frustrated because they can't fight) , this cantrip will be effectively useless for you. If undead were merely resistant this wouldn't be a big deal because cantrips aren't about big damage anyways.
Also a bunch of fiends are immune, but those are higher level so at least those aren't a problem in average games. But undead make up a ton of the low-level monsters.