THE most irritating thing in fanfic to me is a Snape who has no sense of humour. Like, snark is a huge part of his character. Especially when he was younger. This is the guy who invented a spell based around a bad pun on his name (”sectumsempra” pretty much translates to “sever forever”). He is fully capable of REALLY TERRIBLE PUNS, not just biting sarcasm.
Harry compares his humour to his own or Ron’s, if a bit darker. Underneath all of that scowling bitterness Severus Snape is a sassy drama queen to the nth degree. He’s got the emotional maturity of a fifteen year old and you can bet your ass that he’d be joking around like one if he wasn’t laden down with so much stress and responsibility all the time.