Absolutely not. I've dealt with a lot of pessimistic, depressed people--I've been clinically depressed, as well, in the past--, and we can be the most closed-minded, whiny, oblivious people out there. I think that's exactly how depression actually works: one is enslaved, kept away from experiencing the world as it is, by their extraordinarily reduced perspective, which is seemingly unescapable to the depressed individual.
I don't think it's reasonable to conflate negativity--which leads to bad outcomes--with a realistic mindset--which takes into account potential bad outcomes. I find the opposite to be true, in my experience: the more positive and proactive a person is, the stronger they can become--and not crushed by a negative turn of events.