He was obviously a Soothsayer, and he alone had the awareness and will power to reveal the secrets of the vile Witch covens. But could he wear armor?
Kramer the author was a Clergyman not a Wizard, so I guess he could have worn armour. AFAIK he never did though.
(He was thrown out of Innsbruck when he decided to make himself the City's Witch finder. Everyone he accused of being a Witch he had some form of grievance against. Malleus Maleficarum is generally recognised as his attempt to get back at those who mocked him and to validate is own self importance.)