"Die" Muppets would be correct, not because "Die" is feminine, but because it' also the plural form of "Der" & "Das"
Der Mann - Die Männer (the man - the men)
Die Frau - Die Frauen (the woman - the women)
Das Kind - Die Kinder (the child - the children)
therefore:
Das Muppet - Die Muppets.
But who does really care?