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.
Drow as a race arent evil. But their society that worships Lloth sure as hell is. A statblock in a DMG stating them as evil is also not proof that theyre evil. 99 out of 100 drow you will run into however will belong to the earlier mentioned drow society. Which.... yeah, makes them evil. A society that thrives on slavery, killing and offering sentient beeings to gods beeing evil sounds about right to me. Specially considering how much of their own society also suffers from this same treatment.
Same story for the 'orcs are evil' debate tbh. The race itself isent evil. Their society and their actions make them evil.
So no I think you got it right. Some exceptions to this are creatures like devils, fiends, celestials, fae etc etc which are basicly a physical embodiment of their alignment. Those are 'born' into their alignment and its basicly almost impossible for them to change. Because their entire fibre and beeing IS that alignment.