Both are great and over-hated villains but I think Chameleon is better overall.
I am highly annoyed by Chameleon's backstory being reduced to "Oh, she was too smol to do kung fu when Mantis exists" !!
A well known DreamWorks animator explained how her backstory fits with the canon.
I think Kai's backstory has more depth and sounds excellent on paper but poorly executed. Kai is overrated a lot just because of his larger than life action spectacle and theme score in my opinion.
The main reason why I think Chameleon is more well-written is because she is a better thematic foil to Po. They were both prejudiced for their appearances but Po had the support of a spiritual leader like Oogway. They both align with theme of change as Chameleon relied on external transformations to manipulate and assert control while Po and Zhen embraced internal growth. Po and Chameleon are polar opposites as mentors. On the other hand, Kai's arc of revenge and legacy felt disjointed from Po's arc of recognizing individuality and identity. Not to mention no one remembering him is treated like a joke and not really makes me connect with Oogway betraying him. Yes, there are some connections like Po gives chi and Kai takes chi but there isn't really much thematic contrasts between them. Maybe Kai would have been a stronger thematic foil if he had some of his own students and imposed his rigid teachings on them, trying to suppress their own creativity and styles.
I am not gonna into much depth because I know this post would either be ignored or bombarded with disagreements but I am gonna remain firm in my opinion.