25-wire ribbon cables were quite common for serial ports. See, for example, the classic breakout box:
I preferred using cables and adapters. Typically I would use 6-wire cables (ground, TX, RX, CTS, RTS, possibly one more pin) and modular DB-25 (and later DB-9) adapters. But ribbon cables could work quite well if your connections were straight DTE (terminal) to DCE (modem or, in a room full of terminals, some sort of concentrator or multiplexer or I/O box) because you didn't have to swap wires around (you could with ribbon cable, like the IBM PC floppy cable twist, but it was a pain). Easy enough to attach to connectors once you get the hang of it. You could get a big roll of ribbon cable and cut off pieces to fit the actual distance needed and wire up a room like this pretty quickly.