You can do this, but due to file locking, you can corrupt the state if it's shared. jj is specifically designed so that it won't corrupt the repo in this way: https://jj-vcs.github.io/jj/latest/technical/concurrency/
That said, I haven't tried this lately, maybe it's gotten more robust over time. But historically, even a bare repo on something like Dropbox has issues.
I fully agree that this is a niche use-case.
This wasn't what I was talking about, I meant that you should create a bare repo and push to it, not that you work directly in a directory in Dropbox.