Summary
Ash's victory over his mentor Cynthia showed more maturity than his World Coronation Series victory over Leon. Cynthia's guidance shaped Ash more deeply than his rivalry with Leon, leading to his ultimate victory. The battle with Cynthia is the pinnacle of Ash's journey and a true testament to his growth over time.
While Ash was climaxing Pokémon The World Coronation Series battle against Leon may have been his most impactful victory in the universe, but it wasn't the most important for his character. From a storytelling perspective, Ash's victory over his longtime mentor Cynthia is actually a much more important milestone.
Leon acted as Ash's rival, someone who had only just entered Ash's awareness when they first met, and Ash wanted to hit Leon specifically because no one had ever done it. Although Ash and Leon develop a bit of a relationship throughout the series, working together to deal with Eternatus, etc., it's not hard to argue that Ash and Leon's relationship isn't a special one. deeply different.
That's especially true when compared to other Pokémon champions, like Lance, Iris, and Cynthia, whom he's worked with and learned from for much longer than Leon.
Ash's battle with Cynthia helps him develop better
Cynthia first appeared in the Diamond and Pearl episode “Top Down Training!”, where Ash and his rival Paul rushed to meet the champion upon hearing she was nearby. Paul was easily defeated when he challenged Cynthia, but Cynthia turned it into a learning experience for both Ash and Paul, instilling in Ash a desire to fight and defeat her one day. Cynthia would go on to appear more in Diamond and Pearl, helping Ash confront Team Galactic and stop their plans, while also giving advice and helping Ash grow.
Cynthia continues to appear in Black & White, this time guiding not only Ash but also Iris. Cynthia's role is much more of a mentor throughout Black & White, Cynthia saw Ash during the Sinnoh League championship battle, commenting on how she wanted to fight him when he Completely mature as a coach. Cynthia does what she can to help Ash grow in that regard and is happy to help his friends and even his rivals.
This makes Cynthia's return in Journey all the more important; The day has finally come when Cynthia meets a more mature Ash on the battlefield. The fact that he was even standing there showed how much he had grown since she first met him and how he had continued to fulfill the potential she saw in him. Cynthia even showed doubts about her own future at this point, considering giving up the championship before her battle with Ash, but seeing firsthand how far he had come and experiencing the intensity of their battle, she decided not to.
Ash defeating Cynthia in the World Coronation Series was the culmination of a goal he had begun working towards over 600 episodes earlier and ultimately served as a better sign of his growth during that time. than winning the Monarch title. Cynthia was a much bigger influence on Ash than Leon, and being able to finally challenge and defeat her proved once and for all that Ash had grown up since his Pokémon Diamonds and Pearls.