Stupid kiddie theme, none of the kiddie themed pins Stern have made in recent times have sold well, a good example is Shrek only sold approx 500 units 
.
My understanding is that was actually limited deliberately to clear out 500+ excess blank Family Guy playfields/whitewoods Stern already had manufactured. When FG sales were lower than expected they came up with a "Limited Run" of Shrek to primarily clear inventory. This was just as Stern were heading into their 2 year sales slump.
Regarding Muppets, I do agree it is a strange idea for a theme that is for sure. Just way too niche and quirky, even for a market such as pinball.
