I think it has a problem. That is not just fat. It could be egg bound. This occurs when the ovaries are under developed and eggs can’t pass through. You can try this... Raise the temperature to 25 degrees Celsius for a week. Koi usually spawn when the water temperature increases to 20 degrees Celsius in late summer. Raising temperature will help in preparing her ovaries and you will be able to gently squeeze her sides to release the eggs. You should squeeze very gently to avoid injuring the koi. Be very careful.