So... what about "John Denver and the Muppets"? I mean, when you think about it, Lady Gaga is a real person, the Muppets are (technically) not real: nobody is going to want to have billing under something that doesn't really exist. Look at SST even, the live actors are billed before any of the Muppeteers. Likewise on all of Sid & Marty Kroffts' old shows, the live actors were billed in the main titles, and the puppeteers were billed in the end titles.