You have to get your character exposed to radiation damage in order to get mutations in Fallout 76, but what do you do when you get the mutations you want, but also want to get rid of your radiation? Generally you would use Rad-Away, but that's no good if you want to keep your mutations, since this will also remove these.
The only solution for this problem is to make your mutations permanent. This is done by investing two skill points into the perk Starched Genes. This will prevent mutations from being removed upon the character getting healed for radiation damage, so this is the method to being able to keep the mutations without having to live with a constantly high radiation damage.