The Bracers of Defense are a must for Karlach. +2 to AC while not wearing armor or a shield and stacks with unarmored defense. Otherwise, yes, Medium Armor will be better for her until she gets a combined +7 or so in her Dex+Con modifiers. This is a known 5e quirk with Barbarians' AC.
Good to know there will be Barbarian clothing that synergizes with her.
