I think update instead of set is better wording

Can't you just use the plugin in the config dialog to avoid this config madness?

Don't use "emblem" icons - they are meant for "badges on top of files" for instance

I was wondering if we can't just listen for the presence of the com.canonical.appmenu.regisrar service - if the user places an applet *and* has it on decoration, don't think that's too bad, it's a conscious thing? Not sure.

Maybe "applicationMenuAppletEnabled" or something like that?

Please instead of a separate tooltip area set plasmoid.toolTipSubText, ie. above

Plasmoid.toolTipSubText: !useApplet ? i18n("blabla") : ""

Needs to be a slot:

public slots:
    void updateAppletEnabled();

that's why your connection doesn't work

Make it readonly property

Can we make it a binding instead of manually onFooChanged?

Go to System Settings → Application Style → Fine Tuning (tab) to enable it. (I would still love if you could add a way for the user to open that KCM, perhaps in context menu?)

A bit late to the party, but if this is the freedesktop name of an icon, then it should not have i18n.