Having the pick at a slight angle gives you a bit sharper sound.
Experiment... I have used many, many hours playing just palm mutes and experimenting with different pick angles and I like the sound when you pick the string at a 30 to 45 degree angle.
The faster I play the closer the pick is to that 45 degree angle (when I play single string palm mute stuff)
When playing slower stuff with more than one string the pick is closer to 30 degrees.
If you have the pick straight (no angle) to the string the sound will be quite dull.