Nope, it's the opposite. Traffic operating underneath a CTA without Mode C is assumed to be outside controlled airspace, unless radar derived or other information indicates that the aircraft is lost, plus a few other cautionary bits and pieces.
Otherwise it would be impossible to provide any separation within airspace where VFR traffic can operate underneath. Solution: either make permanent operation of Modes A and C compulsory for VFR traffic, or drastically increase the amount of controlled airspace and drop it all down to ground level. Both of which would not exactly help GA!