Counter-examples: I have had expert teachers that were really good, and others that were really bad. I have had non-expert teachers that were really good (at explaining what they knew), and others that were really bad.
Sounds to me that being a good teacher may be orthogonal to being an expert. With the caveat that you need to understand "enough" of what you teach: I have seen teachers giving wrong explanations (in a non-constructive way) about topics I was more knowledgeable than them about.
