So on the other hand there is a toxic negativity that seems to reign in pop culture where people's impression of many things in society in general are on the pessimistic side of what real data shows. I wonder if this tendency towards toxic positivity in social media is some kind of equal but opposite effect that acts as a balance.
I don't think they balance out. I think they're orthogonal. Note that pop culture negativity is mostly about how society sucks; per ineedasername's comment, the protagonist finds love or some other individual positivity. Social media "toxic positivity" is also about individuals having it good, not about the society doing good.
Both phenomena taken together create this negative outlook, "the world sucks and I personally suck". But this negativity is also what drives engagement, so there's no incentive to stop it - not in works of fiction, and not in social media.
I don't see the dominance of negativity in pop culture. Just browsing through the top songs on itunes right now, about half appear to be in some way positive about finding love, overcoming something etc. The other half do seem negative, but that's not dominance, that's 50/50. Then there's other avenues of pop culture, like movies. Literally the most popular movies in the world for most of the past decade have been about people with awesome super powers coming together to do what's right and overcome evil. So again, not really negative.
