Do the 7-zip archives available below count as portable?
https://update.mapeditor.org/snapshots-win/
You should be able to extract them to your flash drive and run them from there. The only part that makes them not entirely portable, is that Tiled will still store its configuration in your user folder (or in case of Windows, in the registry, apart from files like default the objecttypes.xml and any saved tile stamps).