I'm going to hazard a guess that Disney -- who owns the Muppets now -- probably wouldn't risk trashing the whole franchise for a giggle. Sounds like Grover's saying, "Sounds like an excellent idea to me!" It got a bit mangled by the Grover voice, though, and I can hear it the other way, too.
The thing is, when they're standing there listening with a script in hand -- and when they were editing -- they wouldn't hear the f bomb because they're reading the script. Considering how often these things are reviewed -- in production, again in editing, again if there is post for visual effects, again for final sound mixing, and then network standards and practices, it is kind of surprising that no one heard the ambiguity, though! I guess its so unthinkable that no one thought it.