Level 6 Valued Member
The best diet to couple with Kettlebells emphasizes fat loss through calorie restriction or intermittent fasting (both of which are usually coupled with weight lifting in order to increase muscle mass).
I usually do the opposite, a modest surplus, when I do my heaviest KB training, using them during my winter hypertrophy meso cycles.
As for increasing muscle mass during caloric restriction:
It's tough to gain muscle in a deficit, unless you're de-trained or new to lifting, combined sufficient body fat stores.