I suppose you could try and outdo her by setting out outdo her by being even more negative about everything than she is. "Good morning"   "No, it's a horrible morning" - and go on from there.

I'm not saying it'd work, but it might put a different spin on things. Introduce an element of competition into things. Maybe you might even trap her into saying something positive like "I wouldn't go that far".

Of course the problem here is, it's not a colleague on the same level doing it, it's your immediate boss. She might react badly when she realises you're taking the piss out of her. But getting on her nerves might be a way of getting a good reference from her, as a way of getting rid of you.