You need to look carefully at each affected plant and see if there are any signs of infestation. It's not a good idea to spray plants you're going to eat with chemicals, after all! If it's any kind of bugs, then try a spray of soft soap diluted down.

It might be our 'friends' the slugs and snails, too.

There is a little green caterpillar that specializes in mint. They are often easier to see if you go out at night with a flashlight. That is also a good way to catch slugs and snails in the act, though they usually take bigger bites.

Photograph would be useful - if the leaves look distorted as well, it could be capsid bug.