This exactly my experience. I have been a top writer in Food and Fashion. And I am certainly not qualified to write about either. But I wrote humor stories about food and clothing/fashion, and tagged them with Food and Fashion. If you tag enough stories with a tag you get Top Writer in that tag. It is a participation medal.
As you mentioned, I have seen serious essays appear in the Humor section because the writer consistently uses the Humor tag, even when the stories are not humorous. I am confident that if I tagged all my humor stories Bitcoin, I would end up a top writer in Bitcoin. I don’t think it’s ethical to use completely unrelated tags, but as you know, people like “hacks.” If they can get curated by using unrelated tags, some will.
Personally, I wouldn’t put a disclaimer on my stories. I like to surprise people. I don’t want to be pigeonholed as one thing. If readers are looking for a funny story from me and get serious fiction or an essay instead, I don’t think it will hurt them. They may be disappointed, or they may be pleasantly surprised. I hope they eventually learn to expect anything and perhaps even enjoy it.