Leadership is difficult, and what you describe is a common situation. I would start from the positive thing here: Your boss is asking for feedback. In your response, it's great if you can start by stating your commitment to him/her. You should also lay the foundation for an honest discussion.

You could write something like "Boss, I respect you a lot and want to help you become an even better leader. For that reason, I will give my most honest feedback and suggestions. I hope it will be useful. If my comments are wrong or irrelevant, feel free to ignore them."

After that, you can go into concrete suggestions and observations. This is the sensitive part, because inevitably the discussion will circle around weaknesses in your boss's leadership. It's important that you are honest enough to make your contribution useful, but also respectful enough so that no bridges are burned. You will want your boss to know that he/she can trust you and that you support him/her. In your response, it is excellent if you can offer concrete help. For instance, you can offer to help your manager prepare for meetings or write reports.

Of course it is also possible that the boss sent the email only because his/her own boss told him/her to. So it could be that the email was not meant seriously. But if that's the case, then you have a situation of indifferent leadership, and you might as well give your feedback, while you start looking for a job in a team with a leader who cares about leadership.