It’s complicated. AFAIK there is no official ban.

It’s not so much that time travel itself is banned, it’s that the Chinese government has a bunch of rules that make time travel movies impossible to execute.

For example, the Chinese government wishes to protect the dignity and honor of historical culture. And that means, any movie made of those time periods must fundamentally be serious.

Also, it’s possible that whoever is running the movie review board personally thinks time travel is “too silly”.