I loved it. I imagine it really depends on your definition of "dark". Personally, I find shows like "House of Cards" or "How to Get Away with Murder" much darker than shows like "Stranger Things". In fact, I actually don't like watching shows like "House of Cards" because it seems like everyone's always lying and betraying each other and plotting behind each other's backs and acting cruelly toward each other. It just wears me down and makes me feel depressed about the world and humanity in general. I don't like walking away from a series thinking, "Wow, the world really is full of awful people."
"Stranger Things" may have a scary monster and a handful of 'bad guys', but by and large the average person in that show is a decent human being who can make mistakes and behave badly, but whose motivations are easy to understand. And some of them even try to make amends when they realize they've hurt people.
So, for me, it was a much lighter, more optimistic show than "House of Cards". Even if not everyone makes it out alive.