Are things that change in Upside-Down reflected in the real world?

Probably not

Changes in the Upside-Down likely do not affect the normal world—nor the other way around.

1. The Upside-Down does not affect the “Material Plane”

   This isn’t very likely. We don’t have a long list of changes to the Upside-Down to compare, but….

   There are seemingly a number of large creatures moving through the Upside-Down (the Demogorgon, the Demo-dogs, and of course the Mind Flayer). Surely they’d be causing large changes that people would notice, if the Upside-Down affected the Material Plane?

   The Mind Flayer sends vines through to dig tunnels and infiltrate the normal world. Would it have any need to do so if it could affect the world directly from within the Upside-Down? It could simply dig the tunnels there, rather than risk exposing itself. It could collapse the institute. It could collapse a tunnel on Hopper. It has a malign, if unknowable, intelligence—that it doesn’t avail itself of such techniques suggests that they are not available.

   Why would anyone (e.g. Will) have such difficulty communicating from the Upside-Down if changes propagated to the normal world? They could just smash a wall, or carve letters into the ground.

   The one exception, of course, is that people passing through the Upside-Down can affect lights on the other side. However, this is more a unique effect—they don’t have any way of turning the lights on in the Upside-Down.

2. The “Material Plane” does not affect the Upside-Down.

   Contrary to the premise of the question, changes in the normal world likely do not get reflected in the Upside Down. The Upside-Down doesn’t seem to have seasons (we see it during various times from summer to winter, and it looks roughly the same), even though these changes are occurring in the real world. Moving objects, like cars in the normal world, are stationary.

   Nor did putting up the Christmas lights make them appear in the Upside Down. Going back to the episode where Joyce and Hopper were walking through Joyce’s house in the Upside Down in the first season, the Christmas lights don’t seem to appear (though they could be covered by the plant matter). Since they were all over the house, this would be a clear sign that changes to the real world don’t go into the Upside-Down.

   Rather, Will could perceive the Christmas lights from the Upside Down. In Season 2, he’s shown to have developed some connection with the Upside Down from having spent time there, being able to see (and even perhaps enter) into it:

   WILL: You know on a View-Master, when it gets, like—

   MIKE: Caught between two slides?

   WILL: Yeah. Like that. Like one side’s our world, and the other—the other slide is the Upside Down.

   _{Stranger Things, “Trick or Treat, Freak” (S02E02)}

   It’s plausible that his sojourn, or previous innate abilities, or both, enabled the opposite—for him to see the normal world from the Upside Down.

   He was also able to hear his mother talking in the Material Plane, even when Joyce and Hopper couldn’t hear Nancy, Jonathan, or Steve talking during their excursion.

   As for why this is, Will might be a tad psychic. In Season 2 (“The Mind Flayer”), he is possibly able to make lights flicker while in the normal world. While this could be the Mind Flayer’s doing, it could also be evidence of innate talents on Will’s part.

In conclusion, most changes to the Upside-Down have next to no effect on the real world, and vice versa.

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Why is all this? Speculation warning!

The true nature of the Upside Down has not been revealed yet (and may not be until the fourth and final season), but there’s a telling little clue.

DOCTOR OWENS: How’s it looking out there, cowboy?

SOLDIER: Uh, you know, Doc. The usual. Nice and nasty with a chance of radioactivity.

_{Stranger Things, “Trick or Treat, Freak” (S02E02)}

The mention of radioactivity is bit odd. Not that the Upside-Down is a hospitable place, but it’s not even as if whatever radioactivity is there is immediately lethal. It’s more like the sort of thing that you’d expect from…hmm, from a place where a lot of nuclear bombs had been dropped.

I do not believe that the Upside Down is any sort of “shadow” of the normal world, where the effects of one world propagate to another. All that about the Vale of Shadows in the first season was a bit of a red herring. I believe it is a full alternate Earth, one where there was a nuclear war (don’t forget the constant references to Russia, the Soviet Union, and the Cold War in this series). A place like Hawkins likely wouldn’t have been bombed directly, but there’d be a lot of radioactive fallout. And the idea of radiation causing supernatural mutations has been around for a long time….

This also explains why the Upside-Down looks like the town of Hawkins, but has its own peculiar attributes. If it’s a Plane of Shadow, why doesn’t adjust to look like the current world, albeit slowly? What’s up with the positions of cars?

Thus, there is communication between the two worlds—they’ve been drawn closer together—but one doesn’t determine the other, because neither one was directly created from the other.

if you look at the very end of season 2, while the view of the snow ball falls through to the upside down version of the building, you can see the snow ball decorations on the building in the upside down, which means the decorations they put up transferred to the upside down. maybe the upside down updates or something when there's nothing and nobody around to see it?