Joker thinks this is the joke. What can possibly, in any way, matter? It's a universe explicitly build for amusement, so he might as well amuse himself.
To Batman it makes no ethical difference. "Fictional" people suffer just as much, and they matter just as much. Philosophical implications aren't a priority.
It does have tactical implications, though. He always tries to win, always thinks there's a way to win, and he even tells you, us, why: He's Batman.
We just all misunderstand what he means by that (but not the Joker, not him, Joker knows Batman knows they both know).
It's also why he feels personally guilty about every tragedy. It's his book. One way or another, Jason died *because of him*, his parents died *so he would survive them*. Joker or not, tragedy stalks him because that's the nature of the fictional world, that's the nature of Gotham as she's written, but what else can he do? He has to keep trying, save as many as he can whenever he can, plan for the fantastic because that's the kind of world he lives in, take the guilt and the pain because then, maybe, the others will be spared some of it.
Endure the nightmares, the darkness, the endless pain, the insanity, the horror of a battle he knows he'll never be able to win, because that's probably the price of Gotham's existence.