Locations
The Planet of the Mimes
also Planet of Mimes
The homeworld of the Copper Kidd, outside both the Milky Way and Limbo, whose people speak in whistles and tones.
The Copper Kidd is the only member of the squadron who is not from Earth, and the show accounts for him in a single line: he comes from the planet of the mimes, whose people communicate in whistles and tones rather than in words, which accounts for a hero who never speaks and for Bluegrass doing most of his talking.
The world itself is never visited. Everything past that one description, the pointed ears and the facial markings, comes from a single fan wiki rather than from the show, and it has been left there.
Sources: Wikipedia, including its summary of the first Star Comics issue, and thundercats.fandom.