Is "RS-232 Cable" a valid term?
Well both DE-9 and DB-25 connectors are used for RS-232, next you also have RS-232 over 8P8C connector (commonly used for network equipment console), you also have "null-modem" cables for connecting two devices of same type, there are various cable combinations depending on the gender of the connector and so on.

In my opinion, term "RS-232" cable by itself is pretty useless since it carries very little information. I do think that it could be useful in a field where sufficient amount of context is assumed when talking about cable type.

As for term "serial cable", well it's even worse than RS-232 cable. With RS-232, you can at least more or less safely assume voltage levels of the devices, with serial cable you can't. It could be a 3.3 V device or a 5 V device or a device using RS-232 levels. From cable's point of view, those are more or less same, but from device's point of view, the difference is important.

Furthermore, there are numerous other "serial cable" types which would have little in common with RS-232 cable. For example, there's the RS-485 which is also serial, there are various types of WAN connectors for network equipment that are sometimes called "serial cables" and have very little to do with RS-232.