I disagree.
You've got to take into account that the budget this season was very spaced out and divided up, this show doesn't have a multi million dollar budget to make a whole other world like stranger things can and flesh out the environment to their own liking and their own vision, that's like asking the high school stage play to produce a big budgeted broadway play with unique sets and designs. I'm gonna say they went the ST route for a reason and not because they were trying to copy cat it on purpose but probably because they just didn't have it in the budget to do a more elaborate set or setting on the budget they have.
This whole ST comparison in general is just complete bullocks, yes they are similar in concept and how the world looks but the execution for both worlds are very different.
Ones a realm for lost souls that roam the earth endlessly or their souls are consumed by the caretaker of that realm.
The Upside down is a whole alternate dimension wanting to overlap earth.
Side Note
if you think about it, this little in-between realm is basically evil's trash chute for consuming souls and that reaper looking entity is the caretaker who sorts out the souls and feeds the dogs down below.
Plus the color green and the green haze is the perfect color and template for a realm for lost souls. I don't see how it's not fitting for that type of realm.
@MaidOfKandar
I'm gonna say that was a foreshadowing scene for ruby being a dark one OR foreshadowing the dark ones catching up with ruby.