How to fix an electrical outlet where the threaded mounting hole is stripped?
If the hole is slightly stripped, then slipping some type of material in the hole to take up space might do the trick.  A small sliver of card stock or paper or a very thin strand of copper wire (thin like a thread, not thick house wiring).

If that doesn't work or if the hole is actually cracked wide open, you could use glue to hold the screw in place.  Superglue will probably work well and will set fast.  Epoxy is another good choice, but it can be a bit messy.  Even something as basic as wood glue would work to some degree, but that takes a while to dry.  Epoxy will be the strongest choice here.  The other glues would probably fail after a while or if you tried to plug in something with a very stiff cord.

Glue will make it impossible to remove the outlet, but since the box is broken already, anyone that would replace the outlet would just replace the box as well.  They are usually very easy to replace once you've done it a few times.