The answer to this question really depends on the individual and their goals. Generally speaking, 21 percent muscle is considered to be a healthy amount, and many people strive to have a body fat percentage around this range.
For example, the American Council on Exercise (ACE) suggests that most men should have between 18-24 percent body fat, while women should have between 25-31 percent body fat. Therefore, 21 percent muscle would generally fall into a healthy range for both men and women.
However, it is important to note that if you have a specific goal in mind, then 21 percent muscle might not be the best option for you. For instance, if you are looking to compete in bodybuilding or fitness competitions, then you will likely need a much lower body fat percentage. On the other hand, if you are more concerned with overall health and fitness, then 21 percent muscle would likely be an appropriate goal.
Ultimately, the answer to whether 21 percent muscle is good for you comes down to your own individual goals and objectives. If you are looking for a healthy, balanced body composition, then this amount of muscle would likely be a good target. However, if you have a specific goal in mind, then you might need to adjust your goals accordingly.