Fanfic and 'shipping searches live and die by platform culture, honestly. That pairing feels like it's got a strong foothold on Archive of Our Own from what I've seen. The tagging system over there is a lifesaver for niche crossovers like this—you can filter by 'Joker (Persona 5)', 'Kitagawa Yusuke', and then just 'Joker/Kitagawa Yusuke' specifically. I've found some real character-driven pieces that way, where the authors really dig into the dynamic of the artistic observer and the phantom thief.
Your mileage might vary on Wattpad; it's more of a mixed bag. Sometimes the algorithm pushes the most popular tropes, which means a lot of high-school AU fluff, which isn't always my thing. But I did stumble across one a while back that was a slow-burn mystery set in the 'Persona 5' universe post-game, and the prose was surprisingly sharp. Tumblr's where you find the moodboards, the headcanons, and the shorter drabbles, which can be a great way to gauge the fandom's pulse before committing to a longfic.