If someone is about to give a talk, then (depending on the venue) it may be appropriate to say "Stacy is very funny," because it has a clear implication for the situation at hand.
But imagine this. A woman walks by - your colleague Bob knows her and waves. You say, "Bob, who was that?" He says, "That's Stacy from Accounting. She has a great sense of humor." Do we think Stacy is great at her job? Probably not.
The point is that in a professional context (certain professions excluded), it is never relevant to comment on someone's appearance or sexualize them. Like many things related to sexual harassment, the waters can be murky, and many people don't realize they are undermining their colleagues (often people they do in fact greatly respect) by their words and behaviors.
And there is just no excuse for commenting on someone's appearance or sexualizing them in a professional context. Even if you don't know them well.