If you click "Show in Main Panel" in Tag Editor, you can then use tab and the arrow keys to change the track you're editing and its tags.
If you're only changing one field that is shown as a column in the main panel, you can click a value in the main panel and then move between tracks with the up and down-keys. For this to work you need to enable "direct editing of tags in the main panel" (Preferences > Tags (2)).
But I agree, there should be a key, some combination of Tab maybe, to easily select the previous/next track while in the Tag Editor.