The Tavern

If I have several good mutations and one bad mutation, and I convert to Zin, reach 6-star piety, and take a potion of mutation - will that potion erase the bad mutation, before the others? or all of them? or one of them at random? or something else?

You can't get any mutations from mutations potions under zin, so they behave just like cure mutation potions.

According to the wiki, it will remove 3 random mutations. If you want to be certain, try checking it in wizard mode.