i think one of my favorite deacon headcanons is that he only lies:
when the lie is so patently inconsequential that it doesnt even matter that he's lying (ie ricky on the bridge), or
when the lie is so obvious or the person he's lying to knows him so well that it's honestly more of a bit than a lie (like the synth thing. i just know he was gigglin in private waiting for sole to open that stupid note and call him on it).
almost nothing in the game indicates that he's actually a good liar when it matters. in fact, if he really was a good liar, it wouldn't be such common knowledge because no one would catch him in it. des immediately calls him on his lie when he and sole come back from the switchboard, he get flustered when you confront him in bunker hill, hell he barely chokes out a believable lie when he's trying to get clearance for the prydwen.
but because he maintains so insistently that hes such a liar, no one trusts him anyway. his reputation negates any necessity to actually lie, and if a liar knows anything, it's that the best lies are the ones you don't even have to sell.