Is hyperpigmentation in addison's disease reversible?

Yes: What determines the long term pigmentation of the skin the number of pigment producing cells "called melanocytes". In addison's disease (especially if untreated or undertreated), the hyperpigmentation is due to prolonged acth stimulation of the melanocytes, asking it to make more pigment "melanin". Once acth levels come back to normal, the skin shoud return to its baseline pigmentation tone.