FYI, I did some digging, and it seems like you're using the "Mint Y" icon theme based on the discord icon you have:

So one approach that could be taken in the case of Linux Mint would be for Joplin to use the freedesktop Icon Theme Specification and for a compatible icon to be added to the Mint Y icon theme. Similarly, a GNOME-style icon could be added to the Adwaita icon theme. Using a GNOME/KDE-style icon as the default seems plausibly safe, but I can't back that up with, say, market-share data.
EDIT: GNOME does not seem to distribute third-party application icons in its icon theme. Neither does KDE. So, yes, having the default icon fit with both GNOME and KDE seems safest, while submitting specialized icons to distro-specific icon themes could help Joplin fit in on those platforms.