I think WWF Thumb Wrestlers were based on the WWF cartoon Hulk Hogan's Rock 'N' Wrestling. They were basically just small rubber wrestlers that came in 2-packs. I wanted Hulk Hogan and Rowdy Roddy Piper.
You stuck your thumb up into them, and thumbwrestled. Not something the actual Hulk Hogan or Rowdy Roddy Piper would've likely let you do to them.