Fake smoke not likely; fake tweets something else
It would be hard, I imagine, to fake smoke from the Sistine’s little chimney, but faking, say, tweets from inside the conclave seems quite do-able.
Cardinal electors are generally forbidden to communicate with the outside world until a new pope is determined (indeed, they are sworn not to do so) but some are authorized to make certain communications with outsiders (UDG 44, 53), and that’s enough for the veneer of plausibility to attach to fake messages by, say, an unknown implying clandestine access to the means available for such communications.
If some goofy in an ill-fitted cassock and purple scarf can mingle with cardinal electors at the Paul VI Hall, surely someone with brains can produce a convincing tweet allegedly coming from behind the famously locked doors.
I’d ignore them all.