Here's a flow chart of how the fediverse works.
You don't ever see all of the fediverse from one instance. The vast majority of what you see is people followed by people on your instance.
Hashtag searches are limited to that, too.
So if you want to make your bit of the fediverse good, you have to:
1) Boost cool stuff to all your followers on other instances, especially if they contain hashtags;
2) Use hashtags.