This looks like damage from the Four-lined Plant Bug, a common garden pest whose activity is fairly brief because they only have one generation per year. Adults will be finished feeding soon, if they haven't gone already. Plants often outgrow the damage easily, though you can trim away the injured foliage if it's an eyesore. Intervention with a pesticide is not necessary and often ineffective since contact-type pesticide sprays often miss the insects themselves since they are so shy. Fortunately, mint is especially vigorous and a healthy plant should have no trouble recovering completely on its own. (Injured leaves don't heal, but the plant will quickly produce new growth to replace them.)

Miri