You can use "Прохладно" when you're talking about temperature.
If originally you meant "cool" like "awesome" or something you could admire, you need to use word "Круто" or "Классно". I hope it will help you.

Ah, thanks for the feedback! Though in this case it's fine, the character uses a bunch of languages and part of the joke is how she isn't quite right at some of them. Not that I know Russian myself either, I just used Google, so when I wrote that one I knew that there was a chance it could be just for temperature, but that would just make it funnier if someone happened to catch it, which...seems it paid off?