Hi, A user reported on our bugtracker that xfce4-mpc-plugin doesn't have a transparent background, so it renders kind of ugly when one sets the panel as transparent or when using a background image. (I'm not sure it's easily fixable, it's probably the widget used that causes it, so I filed it as "enhancement", feel free to change/ignore that :P). Thanks.
Fixed in 0.4.0, more precisely in http://git.xfce.org/panel-plugins/xfce4-mpc-plugin/commit/?id=efa3bc8d04d5ebe3f8a4beac5d47a41b6bc698a0 / http://git.xfce.org/panel-plugins/xfce4-mpc-plugin/commit/?id=9c5196ef3e0b018562824aa5cc4a6f03f809b1d2