There's two types of Cassandra
The one where she still comprehending about humans, how they interact, how they joke around and often says stuff that seem or sound rude/mean/direct but only because penillians are like that, and she's been trying to correct that and clarify everything.

Or

The Cassandra that doesn't know about that and isn't interested in learning about humans interactions, so she often make rude/mean/direct comments and she MEANS it, not caring about the humans feelings.