Women participating in the defence of cities and castles probably wasn't that uncommon anywhere. It's when they put on the armor and actually join an army on campaign that would be out of ordinary.
Out of the ordinary, but certainly not unheard of. They wouldn't necessarily be fighting, but a woman with a title and vassals might have felt it important to be present for a battle fought by her forces, and armor for protection from stray arrows would be a reasonable precaution even if you didn't plan to be on the front lines. I think I read somewhere that Eleanor of Aquitaine wore armor when she went on the Second Crusade.