Tor is just an encrypted network that runs on top of the Internet. What people chose to do with it is up to them. Tor is not an "Underground" service.
Not per se, but I'm not sure what your definition of underground is, it's typically 'hidden, furtive, secretive'.
Hidden: "Difficult to find"
Furtive: "Marked by quiet and caution and secrecy; taking pains to avoid being observed"
Secretive: "Inclined to secrecy or reticence about divulging information"
So while anyone can use Tor and addresses are usually posted on the open web, the people using it for activities they're wanting to keep quiet are certainly being secretive and trying to avoid attention by Tor's anonymous nature. I don't see an issue calling that side of things 'underground'.