Is UTP of any benefit for RS-232?
Although RS-232 doesn't use differential signaling, so that the cables don't use twisted differential pairs, it is nevertheless advantageous to twist the conductors together in order to cancel EMI. The conductors in serial cable are twisted together.

Conductors don't have to be twisted in pairs. Suppose that the cable contains a RX, TX and common ground line: an odd number. The three can simply be twisted, like a screw with three threads, or braided like a ponytail.  The cable as a whole then has no net loop area for picking up EMI.

Interference doesn't read schematics: it just attacks a loop area, not caring whether that is a differential pair, or a single-ended signal-ground combo.

There is no special advantage of the Cat5 cable over serial cable for RS-232, but it should do the job and you can get long runs of it for low cost due to its popularity.