That's sick can have different meanings, so you have to be
conscious of the context and the way it's expressed.
1; Can be used to express awe or admiration for
something interesting.
2; Can be used to express a disdain for
something not-very-pleasant.
Person A: I got a finally got a new phone!
Person B: That's sick! You gonna give me
your number?
OR
Person A: And he just gutted the fish! In front of me!
Person B: Ew, that's sick!