It looks like that between the two, Joker was worse off after his breakup with Harley Quinn than she was without him. The Clown Prince of Crime's post-Harley life took a dark turn and showed how much of a positive impact she had on the Joker.
When Harley Quinn first came into Joker's life as Dr. Harleen Quinzel, Joker was able to use his charm and candor to seduce her away from a normal life to that of a supervillain. In her earliest days as the Joker's number one acolyte, Harley hung on his every word, always vying for the villain's attention. Joker and Harley Quinn's 'romance' was incredibly one-sided, with Quinn putting much more into their relationship than Joker ever did. As far as he was concerned, Harley was never much more to Joker than a henchman who eagerly went along with any plot he'd fashion.
However, it seems that there was much more to their relationship than the Joker would have people believe. In Harley Quinn #25 by Karl Kessel and Craig Rousseau, Harley Quinn gets involved with Batman in gambit to draw out Gotham's most notorious criminal. Batman is looking for Joker and believes the only one who can find him is Harley Quinn. After they pull a stunt in which Batman publicly states Harley Quinn is his greatest enemy, the Joker does indeed reach out to his former flame. Harley ventures to where Joker has been hiding since the two split up and is shocked at what she finds: a bedraggled Joker squatting in an abandoned bomb shelter. In addition to eating meager rations and drinking questionable water sources, the villain has become utterly aimless, hit with a creative block that's put the Joker into a slump.
It comes as a real shock seeing the villain in such a disheveled state. Joker’s relationship with Harley Quinn was generally portrayed as practically nonexistent on the former's side, with Harley pretty much being the glue to hold what little they had together. But this moment shows that his partner-in-crime's absence from his life had an extremely negative effect on him. As crazy as it sounds, the Joker might have actually needed Harley more than she needed him.
When Harley broke up with the Joker for good, one thing that became abundantly clear was that she was going to be just fine. She had friends, joined teams, and found new love. Harley Quinn positively thrived without her former Puddin’. Without Harley Quinn, the Joker falls apart. He lives the life of a hermit and becomes creatively stuck when it comes to planning crimes. Despite his insistence that he doesn't need Harley, it's easy to see he does. He needs that idolization and blind worship because, without that constant hype, Joker has no one to believe in him. Unfortunately, that's a personal problem for the Joker to resolve by himself, because Harley Quinn is doing just fine in their post-breakup life.