You would think a civilization that holds the morals of violence and deception highly would be a bit better at detecting this.
For that matter, a civilization that at age 13 at least, probably spends more time communicating in IM chats than they do actually talking to each other. (Trust me, it makes sense when you think about it.)
But the point is that Terezi was 'fooled' if it was the case of Gamzee writing that note.
I can't believe we ever thought the trolls were smart.
It's been pretty thoroughly established by now that, however intelligent Terezi actually is, she's an unconscionably bad investigator, if it could be said that investigating anything is ever what she was supposed to be doing. Basically every character in the story that has an attributed skill or specialty turns out to suck at it.
Which brings it back to whether Gamzee's shtick is imitation, what with the wildcard weapon-slot and the "Bard" thing. So far, the only person he's actually "fooled" was Equius, who even in-story is thick as a brick. That said, it wouldn't
entirely surprise me if Hussie really did wind up playing that "most important character in the comic" bit straight and makes Gamzee be Everyone Ever.
As long as we're dropping insane theories, one I thought was pretty funny is that
Terezi+Gamzee is Hussie's own "crack" pairing. Recall: Gamzee was Terezi's server player, and did a remarkably unremarkable job at it; Terezi feels some amount of
protectiveness for him, in a culture where "love" is supposed to be founded in "pity"; and everybody's getting obsessed with ancestry. Which is pretty up in the air with Gamzee, so who knows what's gonna happen.