You can do to your own router whatever you like, I only recommend you not to do it.
There could be a security vulnerability in the FTP server that leaks your password or other sensitive info, it would not be the first time that this happens.
It is not a good idea to expose RouterOS services to the internet unless absolutely required (e.g. VPN services), “just for the fun of it” is not a very good reason…
Keep ftp, telnet, ssh, winbox, api etc inaccessible from internet, and if you need access at least do it via a VPN and/or a filter on fixed external address of an admin.