Guess that depends. Pokemon are sentient, there's really no argument to that given so much obvious in your face proofs of it (though the level of intellegence varies). But just like some people think all Nidos are "lol dinosaurs" then there's always gonna be some misconceptions among those who don't research correctly or discredit stuff just because it doesn't match their theory.
Let's just say, though that Pokemon are only slightly more intellegent than normal animals. Then yes, certain natures would almost dominate the species. Nidorino, for example, would always be irate by default. Like any animal that's quick to attack (the only example I can thing of off hand is Yellow Jackets because I hate them, but that's just hive mindedness on their part, really).
So, since there is no "irate", I'd go with any combination of Adamant/Hasty/Impish/Rash. Nidoqueen, being maternal to a fault, probably Careful, which would also describe Nidoran male since all the Pokedex can do is talk about how big his ears are. =P
Now, looking at it again with the knowledge that even the lowest Pokemon has something similar to human conscienceness (example: Caterpie. Exceptions may be reserved for the extremely stupid creatures), even if minimal, they aren't entirely subject to all being the same way. A lot of it could do with how they were raised, though. I can see wild Pokemon being different than domesticated, sentient or not, because there's still a survival of the fittest deal going on. When everything's provided to them, the way they grow up is usually reflected in the way they were raised (or abandoned), much like a child.
So, on that note, they could essentially have any nature. I wouldn't, however, put it past them to still instinctively have a character trait that relates to the rest of the species, no matter how dormant, even if you raised the creature to think it was something else entirely. But then again, maybe not. An extremely absent-minded Nidoran probably wouldn't have that careful nature, and if he tries crossing a street playfully not minding oncoming traffic, well, that's the end of him.
So, while that may or may not answer the topic, it's still pretty interesting to discuss. Unfortunately I doubt anyone else will jump in on this.