OK, here's the reasons:
Harvey Korman: a razzie award would go to him for being the worst ever guest star; cruel, non-singing, muppet-hating man.
Pearl Bailey: due to her boringness
Beverly Sills: operatic voices are just too oldie moldie, plus UK Spot is too scary for children.
Raquel Welch: too sexy of an appearance in the opening number.
Tony Randall: having someone turn into a stone is just too cruel, even for Miss Piggy.
Wally Boag: due to his annoyingness.
Lola Falana: don't know, just terrible, especially the guest, and the left of Gonzo the Great.