It is.
Though, for installing custom clocks this makes no difference as /usr/lib/advaced-clock-plugin is a symbolic link pointing to /opt/maemo/usr/lib/advaced-clock-plugin.
Only things not in opt are the icons, the desktop files and the "libs" to start the status menu plugin and the control panel UI off.

To be honest the python files for the clocks are that small that optification was not really needed.
Though, while I was at it and merging the packages I also put the directory in opt.
This should provide more flexibility for future extension.