A mount can make cloud-files accessible for any application - as they just think it's a normal harddrive or network location. So yes - you can use Plex. It is a common usecase here.
Applications reading from a mount is easy and will work out of the box on a mount.
If you want applications to write data too, this may require using --vfs-cache-mode writes as this is required for compatibility with more complex writing operations that the OS and applications would normally expect a normal hard-drive to be able to perform.
Just be aware that there are some technical caveats to running Plex from a cloud-drive instead of a local disk. it can work fine - but some Plex settings (like most of the extended metadata scanning) ought to be disabled from running automatically. A lot of these need to actually read parts of each file for the analysis, and some of them will even read the whole file. That's not a great idea to do often/automatically on a large collection over the internet.
This megathread contains a lot of rclone/Plex combo information you may find useful, however do note that not everything will be applicable to you as this is a in-depth guide for a spesific and more advanced setup That Animosity uses. I would focus on the details regarding how to config Plex:
Of course!
rclone sync /local/path remotename:/path
Here is an overview of all the rest of the core commands:
https://rclone.org/commands/
You will find extensive documentation of all features on that same site.
I think you will probably also find this flag useful for your use-case:
--track-renames
It makes it so that you won't have to re-upload everything just because you moved some directories around or gave new names to files. Instead it will just move/rename them on the server and save you most of the work.