I found this while browsing through moviemistakes.com, I'll just quote it verbatim.
All of the members of the Deadly Viper Assassination Squad are named after snakes, however Elle Driver's codename, California Mountain Snake, is the only non-venomous snake of the bunch. This is perhaps a cute Tarantino-esque reason for her inability to poison Black Mamba in her sleep.
That is actually very interesting. I hope that Tarantino wrote it that way for this purpose. It would just be another way to show the sheer genius of his writing.
Yes, his code name is Snake Charmer. But he isn't really a member on the DiVAS, he's the leader. And since all the others are named after snakes, Bill is the 'Snake Charmer,' because he is their leader, and because he 'bedded' at least one of them.
She couldn't use the California Mountain Snake, since, as it's already been pointed out, it's non-venomous. Anyway, it was probably just supposed to be symbolic about how "Black Mamba", one way or another, got them all.
__________________ Evelle: "Balloons! Hey, these blow up into funny shapes 'n' all?"
Grocer: "Well, no...unless round is funny."
What does being cute or psycho have to do with it at all?! All that was being said was that Elle did not poison The Bride, and Quentin made an obscure allusion to that by making her code name that of a non-venemous snake. Simple as that
About California mountain snake (as in the snake, not Elle), I might be wrong, but doesn't it eat other snakes? That'd make sense with Elle character as well, since she did off Budd and tried to kill Beatrix as well.
Also, Elle using a black mamba to kill Budd was a handy way to get off the hook when she told Bill "let's just say he let a black mamba into his trailer.." or something like that. So no blame for her