And "pulling a mug" is a long established phrase (quick attempt at Google-fu was swamped by pottery usage, however, I would have needed to try some more targetted research on that), as is "ugly mug" (you don't see this face-synonym partnered with a nicer term, as you might do in "fair visage", though note that either variation can be used with deliberate irony as well). "Indeed, to mug (up)" is to gurn. But note confusion with criminal mugging (theft by assault) and theatrical mugging (exagerated physical over-acting), either of which may or may not be connected; or have become so later on via some false cognate.
I mean, the history is known, you don't have to speculate. Mug as slang for "face", recorded by 1708; "mugging" meaning to make an exaggerated expression (not
necessarily an ugly one; a self-important actor can 'mug for the camera' without any implication of "gurning") as theatrical slang by 1855, developed from the former. A mug could also mean a mark or dupe in thieves' cant by 1851, probably unrelated. To mug as "to punch in the face" in boxing by 1818, which is absolutely because of the 'face' meaning; extended to general beating-up by 1846, then the modern crime of street robbery by 1864 which is probably a combination with the thieves' slang I just mentioned. People do study these things. The idea of a mug being an ugly or unpleasant face is an undercurrent throughout, but it's never mandatory, can be ironic, and, most importantly, the meaning as a noun
certainly does not derive from the verb, and there is
certainly no connection to the idea of criminals intentionally gurning in mugshots, because
there's no reason to think such a thing ever happened in the first place (even your recent link isn't an example because you can only speculate about his reasons for making the face; since it obviously doesn't actually fool anyone, it's just as likely if not moreso that he just thought he was looking tough), and besides,
the word "mug" was already in general use in low slang before photos of criminals existed. You would need a time machine for there to be a connection.
And so the suggestion of an origin (folk-etymology?) was once made, as I conveyed it onwards. Perhaps "it is said" just doesn't travel quite as well, as a phrase? So I'll try not to use that in future, if it can be so misconstrued.
No, I fully understood that you meant it was just something you heard, which is why I told you that, I don't know where you heard it, but it is not a thing. You heard wrong.
But saying that something is said doesn't mean that it doesn't matter whether it's wrong, and certainly, if you are going to
go on defending it with a wishy-washy, you know, maybe it could have been, it is a mystery, who can say, you cannot also expect to distance yourself from the statement by saying you were only reporting what you heard.
Besides... your theorized connection to physiognomy, which is also definitely wrong, wasn't something you said you were just passing on. You said it as if it were a fact, even with "of course". I don't really get this half-in-half-out approach, either you stand by your statement or you don't.
ETA: I mean, to be clear, if you don't stand by it, that's fine. You are not expected to justify something you once heard and have no basis for possibly knowing anything about. But in that case, why this extremely defensive reaction to being told that it was wrong? You can just say, okay, I can't vouch for it, it's just something I heard once. Instead, you're acting like my saying it's wrong is somehow misunderstanding your intent in bringing it up. If you say you heard something once, then you must know there's a possibility people will tell you you heard wrong.