Certifications are only important to the people that hiring you. If you want to work for a tennis center or facility then it is important. If you are independent, it is less so.
Almost all the players I know that are looking for instructors don't know or don't care about certifications. It is all word of mouth of who is good at teaching and who gets results.
I know plenty of instructors that are certified at the highest levels but are not good instructors and some are surprisingly marginal players.