Not sure what the experience is like when just using the GUI on a normal distro. But on my arch setup I just installed fcitx5, fcitx5-mozc, set up the environment variables to actually use fcitx, and make it autostart, and I had japanese input working just fine.

I am guessing that most normal people desktop distros like popos just have something like fcitx set up. In which case you'd just have to install the plugin, or, I wouldn't be surprised if it comes preinstalled on some of them.