Sometimes.
In many games I run it's impossible to objectively describge someone as evil, so that is not a good criterion.
I stat NPCs that are important enough that the PCs will interact with them a lot (not necessarily by combat - in many non-combat conflicts stats are no less necessary). I also stat these who have a unique, mechanically represented, ability or quirk, that would be hard to improvise.
For most other NPCs I have a note on how powerful they are and what is their area of expertise - the numbers I may improvise when necessary.
In many games I run it's impossible to objectively describge someone as evil, so that is not a good criterion.
I stat NPCs that are important enough that the PCs will interact with them a lot (not necessarily by combat - in many non-combat conflicts stats are no less necessary). I also stat these who have a unique, mechanically represented, ability or quirk, that would be hard to improvise.
For most other NPCs I have a note on how powerful they are and what is their area of expertise - the numbers I may improvise when necessary.