Perhaps it is a variety of mint called a Polo mint?
It does sound like something has taken a liking to your herb. The only pest of mint I know about is Mint Moth, but that is not that common and it tends to leave the tips of the plant twisted and distorted. The holes are not going to male the leaves unusable though, as long as you wash off any pest before use.

There is a mint beetle that eats small irregularly shaped holes. The beetle is quite small and is a pretty iridescent green colour. But it is very early in the year for it to be that - have a look and se if you can spot any. The best control is to remove them by hand - they rarely do anough damage to be serious.