Clearly the crotch of a thong wearer is not a naked crotch, but the ass of a thong wearer? What say you?
I say it’s a naked ass.
A string up the crack does not un-naked make. That said, I don’t know that a V of material too narrow to use as a blindfold or bandage counts either.
If it’s like most thongs, buried by the butt cheeks and not visible to the observer, then yes that butt is naked.
“a buttocks”?
I agree that bethonged buttocks are naked. But the OP confused the issue by referring to the ass. An ass is more than just a pair of buttocks. It is the whole unit which includes the buttocks along with the crack and the anus. So an ass can be covered by a thong (at least key portions of it anyway) even when the buttocks are naked.
Ya can’t cover an acre of ground with an eight foot by eight foot tarp.
Is a person wearing just a thong naked?
No, their not.
There butt may be bare, but they aren’t naked, in my opinion!
A bethonged butt is not a naked butt, IMHO. But I agree it’s a very thin line.
Wait, are we talking about men or women??
Why the floop would it matter? Does a man’s thong and women’s thong cover a different amount of butt? In my experience (and admittedly I don’t have much with women’s thongs), it’s just a string up the buttcrack either way.
That’s my view; they are bare, but not naked.
I would say the buttocks are exposed, not naked.
Planning on doing a bit of buttock-swatting?
They have to pixelate thonged buttocks on TV, so they’re naked.
T’aint naked if the taint is covered.
Little Nemo has the right of it. The buttocks are naked, but the ass as a whole is not.
So are they “bare-ass” or not? 
My, what a good, thought-provoking question. I may need to spend some time studying the situation.
“Honey, will you put on a thong & nothing else. No, I’m not being a horndog, it’s for research purposes.” (Gawd, I love the Dope!) 
I thought surely there’d be a relevant Slug Signorino illustration somewhere, but I wasn’t sure what to search on.
What you did there? I saw it.