Them making money off is supposed to justify a poorly done story element? People on this very board spend pages s***ting on all types of movies that make more than 3m...so that argument is moot really.
They never should have included the part about her age when the Joker first appears. Because that puts her at least 14yrs (being generous) younger than Bats and definitely adds to the fuel of him being someone in a mentor capacity, thus adding a power differential to their relationship that makes it very hard not to really see Bats as more than just a "bad parent".