For the love of Harley, this epilogue feels like a little valentines add on after the already cold winter experienced in the main narrative. It snows a lot in arkham to say the least, not quite up there with Arkham Origins but you get the point.
So Harley Quinzel has batman hence Harley's revenge, leaving us the gamer to view gotham through the eyes of his trusted sidekick Robin. Balloon popping along the way just for the hell of it, cause why not?
We also get some more batman in a flashback two hours or so earlier. It feels just like a filler with not much purpose, it's technically a Harley Quinn appreciation add on mourning over her loss of the Joker.