Pollyanna's are the most annoying and fake people around. I'd rather be with Grumpy people who are factual, realistic and capable, than the "its always a beautiful day" Pollyanna crowd...
It's possible to be positive and also be factual, realistic, and capable. Being positive doesn't mean ignoring reality and glossing over everything negative. I'm a fairly positive person, but I definitely still have conversations where I say, "Yeah, this totally sucks, I'm not happy with it, and I think it needs to be fixed".
Approaching things in a positive, friendly, and collaborative manner has always served me and my teams far better than being grumpy and pessimistic about everything.