This show had a weird insistence that the Joker never, ever, ever should have "the" in front of his name. It was always just "Joker." The same was true for everyone else who had a "the" in their name, (Penguin, Riddler, etc.)
It's probably because they knew their cartoon was pointless given Paul Dini's cartoon. So they tried to make it different. And instead they made it horrible. I like that they even had a worthless clone of The Harley Quinn.
There was one scene with The Batman's version of Quinn (in an ep written by Paul Dini) I liked in spite of myself-- a sendup of the "musical romantic montage" featuring a good Hank Williams Sr. song and a squirrel-poisoning joke that Dini'd probably wanted to use in the original BTAS if not for that era's tougher BS&P.