So in the way we use the word "meaning" to talk about other words in everyday English, I think it is quite proper to say that Cassandra "means" "prophet of doom who is not believed". As a matter of fact, one can call someone "a Cassandra" in English with precisely that meaning.
No. When one calls someone "a
Cassandra", one is making
a reference to the mythological character, not using a noun that
means "the characteristic associated with the mythological
Cassandra".