If you emailed something like this about a coworker to everyone in the company, it would also be inappropriate for the workplace. Just because it was the CEO doesn't make it acceptable.
Actually, yes, yes it does. There are some things you can't say to any employee of any rank: racist or sexist harassment for example. And commenting on the performance of an employee that doesn't report to you is also generally a no-go. But legitimate, job-related criticism of the CEO, or any other senior management, is entirely acceptable. Why wouldn't it be?