There's no rule that would result in your challenge being closed for being too complex (although a complex spec is more likely to be unclear, which is a reason to close). However, for any challenge type it helps to make the spec as simple as possible without being ambiguous. Condensing the required information into a form that can be absorbed as easily as possible is an art and an interesting challenge in itself.
This is true for any challenge type, but it applies particularly for King of the Hill contests. In a golf challenge you can continue to improve your score, competing against your past self, even if there are no other people answering. In a KotH a single answer has nothing to compete against, so the challenge can only be meaningful with multiple answers. Ideally the competition should be easy to enter, so a population of players can build up.
My own view of KotHs is that they should be trivially simple to enter, but allow open ended opportunity for further improvement, to allow a variety of approaches followed by an arms race driving innovation. This isn't a rule though. It's not impossible to make a tricky-to-enter KotH work well.
If the input format is likely to be an obstacle to entry, it's worth thinking about how much of it is essential to the core of your challenge, and how much could be simplified. Can your KotH controller do more work so the players don't have to? Some KotHs provide utility functions available for all players to use. You could also provide input to the players in 2 or more alternative formats.
Whether these approaches are useful will depend on the nature of your particular challenge. For working on a specific challenge I find the challenge sandbox very helpful, in general but particularly for KotHs. The questions people ask about the spec and the input format will give you an idea of what needs to be better explained, be simplified, or have examples. It will also help detect edge cases that might break your challenge, allowing you to fine tune the rules before posting to avoid disruption once the challenge is live.