Are aftermarket power cords required to be UL / CE approved?

The National Electrical Code requires that all items installed in a building be tested by an NRTL, generally that means UL listed. This does not apply to your personal use items, but does apply to any fixed appliances or electrical equipment that is installed within your home or commercial facility. If your local jurisdiction has adopted the national electrical code then this requirement applies.

Your stove example would qualify as an installed appliance, so yes it would be required by code.  If you hire an electrician to do work in your house, the parts he uses would become part of the structure and also need to be UL listed.  Anything you plug into a wall socket is a personal use item and there is no requirement that it be UL.