The innocent must never suffer punishment unjustly.
Tips
usage
蒙 here is the verb 'to suffer / to bear', not 'cover' or 'Mongolia'. Same shape but doing different work. Compare 蒙难 (méngnàn, 'to meet with disaster'), 蒙骗 (méngpiàn, 'to be deceived'). 冤 alone means 'injustice / wrong / grudge'.
register
Formal, legal, journalistic. You'll read 蒙冤 in court reporting and human-rights writing. For everyday 'I was framed!' people say 我被冤枉了 (wǒ bèi yuānwang le).
