Definitely works!

Not many clients really have a "popular hashtags" section. I think Ditto.pub does, and the "Topics" tab in Primal's discovery section is actually just recently popular hashtags, too. Outside of those two, I don't know of any others.

Best thing I can suggest is thinking of what tags someone MIGHT use for a topic you want to discuss and search for that hashtag to see if anyone has been using it already. If they have, chances are that others will look for that topic using that hashtag, too.