Native speakers of English are better ESL teachers because they know the intricacies of the language better and their vocabulary is more robust. Non-native speakers of English are still good ESL teachers, but they don't have a lifetime's worth of background in English. A native English speaker may have trouble relating to someone else's non-English native language, but that barrier can be overcome with a translator or another person in the class who speaks the same language.
Yes, native speakers of English are better ESL teachers, because it is important to learn regional dialects. Those who are learning English as a second language are not learning it in order to pass an exam. They are learning it for everyday life. It is important that they learn from someone who speaks English as a native language, so they learn accents, dialects, and local slang.
It goes without saying that to teach ESL an instructor must be proficient in English. There are also many tests that measure English language proficiency: IELTS (band 7), DLI, CEF. Even the US federal government requires its employees to pass proficiency tests in English. It's time we put aside this whole notion of native speakers being the best to teach a given language. In fact, many native speakers of English wouldn't pass stringent proficiency tests; they'd fail to score 7 on an IELTS. Many native speakers are also clueless about teaching methodology, classroom management, theories of language acquisition, or even basic grammar points. Proficiency in the target language, together with teaching qualifications & experience should instead be the goal in hiring teachers for the ESL industry. Native ability is at best a descriptive category. At worst it's a racist code that discriminates in favor of white, blonde, blue-eyed, loud-mouthed passport holders from the Anglosphere (US, UK, Canada, Australia, NZ).
It goes without saying that to teach English as a Second Language (ESL) the instructor must be proficient in English. There are also many tests to measure English language proficiency: I.E.L.T.S. (band 7), the D.L.I., C.E.F., etc. Even the US federal government requires employees to pass proficiency tests in English. I also wouldn't hire ESL teachers based merely on their native speaker abilities. We should focus instead on proficiency, which is measurable. Teaching qualifications, experience and even professional ethics are also important considerations in ESL teaching. Scandinavia is a good example where most ESL isn't taught by native speakers. That's because Danish, Norwegian or Swedish ESL teachers are highly proficient in English. Let's cut the native speaker myth & get on with the business of focusing on proficiency when it comes to hiring teachers for the ESL industry.