That is awesome, I'll shove it onto my system when I get a chance.
From what I can see of that mint-y repo you need to submit a version for the various sizes (in mint-y-icons/usr/share/icons/Mint-Y/apps at master · linuxmint/mint-y-icons · GitHub) - unless there is a generator script or something is run to create those automatically in the directory.
I'm not entirely sure but I guess that because the file name is 'joplin.svg' and therefore 'joplin.png' for the actual icons then you don't need to add a manual symbolic link to the repo, it would handle it natively.