The franchise isn't performing better – based on what, the latest TV show?
Muppets are an institution just like Mickey Mouse. They're not going anywhere. Mickey Mouse doesn't have a television show in prime time. The Muppets don't have to prove themselves every three years to be viable.
Of course they do. "Viable", to any company, is synonymous with "profitable". In those terms - heck, in ANY terms - it's ridiculous to compare Mickey with Muppets. Mickey still sells TONS of merch. Muppets? Not. Even the DVDs of their relatively successful Segel movie didn't sell well - Robert Iger admitted it himself.
The franchise isn't performing well in terms of their last flop movie, failing TV show and lousy merch sales. Hey, you asked.
As for Mickey, currently, he's got a VERY successful series of Emmy-award-winning new short cartoons airing on TV. AND of course he still sells tons of merch. I hate to break it to ya, but Disney didn't buy Henson's puppets with the idea of building a shrine to them. It expected them to be profitable. They're not. Period.