no, do pacifist first, otherwise genocide is really boring. Doing genocide first will have you feel also nothing because you never had time with the characters in general and will just end up playing as if you are spoiling it for yourself.
Though it's a pick-your-poison situation since there's true pacifist which you should also do afterward.
Anyway, if you ever DO decide to do genocide, I'd reccomend not dropping the Burnt Pan from Hotland, you'll need it for a boss that's in a certain corridor waiting for you :)