Close Steam first so the Steam cleanup sticks: Steam rewrites its shortcut list on exit.

Windows: Settings › Apps, or the uninstaller in the install folder. It removes the program, shortcuts, the start-at-login entry and the profiles you added to Steam, and leaves your data folder, SMAPI and the game folder; delete %LOCALAPPDATA%\Mortar to remove profiles, mods and settings.
Linux: first run mortar --release-links && mortar uninstall-cleanup (AppImage: ./mortar-linux-x86_64.AppImage --release-links && ./mortar-linux-x86_64.AppImage uninstall-cleanup, or the aarch64 file; Flatpak: flatpak run tech.rethunk.Mortar --release-links && flatpak run tech.rethunk.Mortar uninstall-cleanup). It gives nxm:// links back and removes the extension's connection, the start-at-login entry, profile shortcuts, Steam profiles, and what an AppImage or portable binary added to the menu. Then remove the package, AppImage or Flatpak. Removing a .deb, .rpm or Arch package with sudo runs those steps for your own account only. Delete the data folder to remove profiles and mods.