Are these beetles harmful?

They are almost certainly leaf beetles (a distinct family within the group). They have larvae that are more caterpillar-like than usual for most other beetles.

Some of this family are notorious! Lily Beetle and Colorado Potato Beetle.

Yours will not be as bad as that but may well be munching. Pick them off and look for any small larvae next to damaged parts of the leaves.

There is a mint beetle (Chrysolina menthastri) which is a metallic green colour. As your patch of mint is 'large', I doubt that anything will do much damage to it in the long run, as mint is so invasive. Took me half a day to clear a patch of roots this spring!