Is keto good for cycling?

No, the keto diet is not as effective as higher carbohydrate eating approaches for cycling performance [1]. 

The keto diet is a high fat, very low carbohydrate diet, where carbohydrate intake is typically restricted to 30-50g per day. Glucose is the preferred fuel for working muscles, and cyclists following a keto diet have less glucose available to fuel high-intensity exercise and increased oxygen demands. Consequently, research suggests that keto negatively impairs maximal performance [2]. 

However, if you’re cycling below maximum effort, the ketogenic diet may be less detrimental to your performance. A multi-study review found keto-adapted endurance athletes were able to maintain moderate-to-vigorous intensity endurance exercise while adhering to a keto diet [3].