Colonel Jorgen:
Fat Freddy's Cat for instance started out in the "underground" comics scene of the sixties, but I do not think you could call it "dark"
I agree that Gilbert Sheldon's work tends to be too comedic to be generally called dark. Perhaps some better (or wider) definitions of "underground comics" would be those that are in some way unrespectable, anti-authority or anti-conservative, and celebrative of counterculture (eg: containing non-condemning portrayals of recreational drug use). Some of that at least would apply to Gilbert Sheldon's strips, wouldn't it? (I don't know them that well to be honest.)
Whereas Tintin, as I think tintinsgf is saying, is usually considered too respectable to be described as underground, even in countries where he's not been considered mainstream or commercial.
Of course, here in the UK, the New Yorker cartoon's caption doesn't
exactly apply since "the masses" (or at least almost everyone over thirty maybe) already knew who Tintin was before the movie came out. And that's even more the case in much of the rest of Europe. But perhaps there's still a certain high-priced non-mass-market caché to Tintin even in countries where he's well known. And in any case, the New Yorker cartoon certainly puts its finger on a general truth about a certain kind of cultural snobbery that's applicable to many things in any country.