Cassandra doesn't seem to have a cold heart to me and as for the tropes, which specifically do you think she fits?
It's not so much the cold heart than the emotionally distant and seriousness. Cassandra appear cold to people, she isn't necessarily.
Cassandra is has a bit of an Iron Lady, except it's not about politics (unlike Vivienne who is 100% the Iron Lady). She is definitively a Sugar and Ice personality and got a bit of Jerk with a Heart of Gold going. And like I said, I see a bit of Snark Knight in her too. DA2 interrogation can be seen as a Defrosting the Ice Queen as well (TVtrope list it as such in fact). She start all cold and business-like and end up on a softer note.
But yeah, she doesn't fit the classic trope of the Ice Queen, because the lava is boiling quite close to the surface in there. She doesn't even fit the classic tsundere trope, even if people list her as such.
@Wolfen09
I agree, I actually find a lot of similarities between Cassandra and Morrigan, except they are sort of in totally opposite at the same time. Cassandra is a like an anti-Morrigan: same view on power, totally opposite goals as basis to gain such power.