Considering a substantial number of the best coaches for professional sports teams are fat middle age men who absolutely could not compete with the players they are coaching, yes. In many cases, you absolutely would hire someone who cannot practice what they preach because they have a better theoretical understanding of how to succeed than you do.

In the case of language, knowing the language is a prerequisite to teaching it because if you have a theoretical and comprehensive understanding of the language, then you can speak it to at least some extent. Knowing is the entirety of the battle. In the case of a variety of other fields, you can have a pristine understanding of the theory without necessarily being able to put in the work to apply the findings of that research in your personal life. Here, knowing is only half the battle.

Psychology has a substantial body of research empirically backing a variety of methods and practices, much in the same way medicine does, and a therapist can advise you to attempt these treatment options on the basis of that research.

This is why I used the doctor analogy, because mental health care is directly analogous to health care. My Doctor smoking cigars and drinking way too much alcohol on off hours does not make him telling me that if I don't quit smoking I'll probably die of lung cancer and that my alcoholism is killing my liver any less valid. He has a body of science to back up the claim, I don't need to rely on his personal character or experience to validate that advice.

Obviously, it would be ideal if you were perfectly put together in your personal life as a therapist, which is why most therapists are directed to go to therapy so they can get other people to identify the dysfunctional shit in their life that they cannot see in themselves. However, this is true for literally all professions.

If you met a doctor who wasn't perfectly healthy, would you feel the same way? Giving advice based on empirically validated studies and implementing that same advice in your own personal life after emotionally draining yourself by spending your entire time at work dealing with other people's emotional issues are two entirely different things.