Originally Posted by Blitzkrieg Bop
Maybe Bolland didn't want to spoil the big reveal in a murky puddle.
Yeah, it was obviously done for a thematic affect for the audience.
By making the puddle murky for the reader, it saves Joker's actual reveal for the bottom panel of the page. Naratively, it's pretty obvious Joker saw himself in the mirror; that's why he's laughing hysterically.