My world has a few large areas of mistlands with no mines, but most of them have several, and many of those are along the coasts. Sailing around the shoreline before attempting an on-foot expedition might allow you to find some easier-to-access mines - it did for me.

If that fails and you don't mind cheating a bit, you can check your seed on https://valheim-map.world and see if your Mistlands are indeed devoid of infested mines or you're just not finding them due to the poor visibility mixed with the difficult terrain.