Just put less force on your palm, and move your palm so that it's on every string seperately right above as well when you're strumming palm muted. If you keep it in one place one string is going to be more damped than the other thus making it sound crappy.
How do I palm mute?
The palm mute is a playing technique for guitar and bass guitar, executed by placing the side of the picking hand below the little finger across the strings to be plucked, very close to the bridge, and then plucking the strings while the damping is in effect. This produces a muted sound.
Why do my palm mutes sound bad?
Chances are that your palm muting will sound pretty bad the first time. The only way to remedy this is to practice. You should be able to hear the pitches of the notes you're dampening. Try picking the strings one at time to make sure you can hear all the proper pitches rather than a dead plucking sound.
How do you palm mute while standing?
put your hand karate chop style. place it on top of the bridge where the strings bend over, place the fat part or what you would use to karate chop a brotha. move hand over slightly onto the strings, more onto the strings or more over onto the bridge will change up the palm muting sound.