Just going to say that a three episode extension is not a full season for the big channels, and seems to indicate that Muppets will not be returning for a Season 2.
At this point, networks will begin making decisions as to what is going to get a shot at a new season next year - those shows are going to be getting upgrades to a full 20-22 episode season (Quantico on ABC, or Limitless on CBS are good examples of this). For shows that do not get an episode upgrade to that total (Muppets is only going to have a fifteen episode season), it often shows that the network is not impressed with ratings - Muppets has seen a sharp drop over the past four weeks.
Only thing that would help The Muppets is if it was contractually obligated to have a shorter season. Some shows, like How to Get Away with Murder, are renewed with shorter seasons because they were slated that way from the start.
But yeah. I don't foresee a Season 2.