In current lore, they are not. But they used to be.
Rly? O_o
Just to be clear i come out of that deffinition we were provided here by Niara (i think? Not sure tho) that creature who is evil "by nature" ... dont have any other choice but to be evil ... and if they somehow manage to "not being evil" they transforn into their oposite ... Fiend into celestial ... and vice versa. Metalic dragon into chromatic ... and vice versa.
Thats why i have problems imagining any creature with completely free will would be evil (or good) "by nature" ... I can easily imagine they tend to one side like half-orcs who have this inherent predisposition for being evil and they either embrace it or fight it. Drow following the same rules are certainly evil bcs they are culturally raised that way, just as Githyanki, but they should be physicaly able to resist it ... and therefore are not evil "by nature".
Feel free to corect me, this is just how i understand it.
I still dont understand why cant we change Race for our hirelings. Lets us play Githyanki as racist as they trully are!