Are robes considered light armor?
No; robes are clothing and are not considered to be armor, light or heavy. As such, robes will not increase your light armor skill.
Robes and other apparel which do not have the armor tags - are not armor. As a consequence, these items:
- do not grant armor skill experience
- do not benefit from armor perks or skill
- do not grant hidden armor rating
You can still get some of these benefits while wearing robes if you wear some armor in other slots (head, feet, hands, shield)