Whenever I use a different icon set/theme, the quit icon on the panel becomes fuzzy. I've tried copying the stock_quit.png/svg icon from the Rodent icon set to the set I want, and the reload the icon set, but it fails to work as well. I'm guessing that this shutdown button is customizable via the icon sets like all other icons are. But the shutdown icon location should also be customizable like Launcher icons are, as wel..
The fuzziness has nothing to do with the panel, most likely depends on the icon theme (does it have sharper small icons, does it use svg, things like that). Changing the icons it not in the scope of the plugin, we simply use hard-coded icon names some some places. You can always create a launcher that spawns xfce4-session-logout.