Can I ask any sifu around here, when I palm mute and strum, the thinnest E and B string tends to sound not muted. Does the bottom 2 strings need to be muted or not?

Sounds like you didn't palm mute it normally 6 string guitar palm mute riff only using 3,4,5,6 strings. There are some going further by dampening 1,2,3 string with piece of cloth to avoid picking them.

depends on the riff itself mayn! like if its required to play that two strings muted then mute it if not , make sure u dont strike those strings.

This.

Basically, if it doesn't sound like what it's supposed to sound, you're doing it wrong.

normally palm mute only involves the low E and A string.

try to get that trademark 'chugging' sound..

You only need to strum the strings that you will be playing, not all 6 strings like you would when open strumming.

alternatively, u can mute with ur left hand fingers.