If I'm remembering this correctly, the main explanation that was given for the council for wearing robes was that they needed to disguise their true identities, even from the Yeerk Vissers, in case of betrayal and assassinations. I believe this was told in the Visser Chronicles during the trial scenes.
Other than that, I don't think the Yeerks particularly cared for uniforms or clothing for their controllers. The only thing they got close to for uniforms was, as Phoenix said above, the armbands which were used to signify which Hork-Bajir guards belonged to which Visser.
Come to think of it, it would've been a smart move for the Yeerks to have the Hork-bajirs plated with some kind of armor, especially in the Yeerk Pool, when they were likely stationed incase of an attack.