The Muppets have always had their pulse on pop culture. They had modern starts on The Muppet Show and Muppets Tonight, as well as their own witty banter in their own spoof films. I don't think appearing in ads is the end of their integrity.
Don't confuse the Muppets with Fraggle Rock or Seasme Street, which are done for storytelling and education, respectively. The Muppets have always been telling their stories around the premise that something ELSE is going on, be it a commerial or guest star.