MATE has it's own power manager and xfce4-power-manager shouldn't start in MATE desktops (just like it doesn't in Gnome/KDE/Unity currently. Please add MATE to the NotShowIn line in the desktop file. Thanks.
Fixed in git master and will be part of the next 1.6.x release: http://git.xfce.org/xfce/xfce4-power-manager/commit/?id=bd40ec1fa381df94d46fdc4b5cf8632ed30b24c9
Thank you, can you do this for power-manager-settings too? No need that the setting GUI displays in mate menus.