"Mouth" represents what a person says. Opening one's mouth means to speak. Alternate translation: "he did not protest" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Isaiah compares the servant to a lamb and a sheep to emphasize that he will remain silent as people harm and humiliate him. (See: [[rc://en/ta/man/translate/figs-simile]])
This can be stated in active form. Alternate translation: "just as a lamb is silent as a person slaughters it" (See: [[rc://en/ta/man/translate/figs-simile]] and [[rc://en/ta/man/translate/figs-activepassive]])