Looking at the Giannis-Jaylen play on Jaylen's dunk attempt, I don't think it was "dirty," but it's probably "reckless" and uncool.
You can see how far away Giannis was as Jaylen gathers:
Most players just let Jaylen dunk there, but Giannis is super-freak and thinks to himself, "It's the playoffs, I have to keep guarding." So he reaches out an arm to try to block the shot as Jaylen goes up:
At this point, he should and probably does realize that there's nothing he can do, but he still lays an arm into Jaylen's side. Does he have to because he's already put his arm out and he's just bending it away from Jaylen? Is he lightly shoving him trying to affect his ability to finish? It's hard to say.
But, to me, with this finish, it does seem like he's actively pushing him, if even just a little, and doing it on purpose:
And then, at the end, he seems to be explaining to Jaylen like, "hey, I bent my arm, I didn't push you."
Then he tries to put a hand on his shoulder and Jaylen tells him to fuck right off.
Jaylen seems to be thinking, "If you pushed me even the slightest bit harder, I'd be lying on the ground with a concussion right now." Which is probably true. And Giannis probably thinks he's so in control that never would have happened.