That's their prison uniform--just enough to preserve some modesty, but not enough to really let them conceal any contraband, and they can't use excess fabric to make weapons or anything, and it means they're mostly exposed. That's an important aspect to keeping the prisoners under control because they'll feel naked and vulnerable, and because it might be a bit cold outside of their cells without anything to wear besides shiny undies and handcuffs.

Their ways might seem strange, or even somewhat cruel by our standards, but they're alien space jerboas so they do some things a bit differently than we do here on Earth.