I don't think what we see in TOS/TAS is the entirety of the Enterprise's "5 Year Mission". Other deaths could have occurred between episodes. So there's no way to accurately calculate what the casualty rate was. "Insufficient data" as Spock might say,
The numbers do show that security is more deadly than other jobs.
Of course it's an exaggeration, but one based on what the show presents. Security are nameless extras for the most part and therefore cannon fodder. Away to show that the situation is serious and dangerous for our heroes.
Is it a cliche? Yes, but not one unique to Star Trek, though Trek has given it a name.