There are some vendors (technology and training/recruiting/consulting companies) that have offered certifications upon completion of training courses for some time now. There is clear value in the training courses.
I have been asked many times over the years by job hunters if there are certifications that would make their holders more employable or offer an advantage in some way to the process. My answer has always been no, that I have never seen a position announcement that asked for any such certification.
The Customer Success Association has published a CSM Competency Standard, Basic Level that lists the things that an individual Customer Success Manager should know, use, and be able to comfortably explain at at need to any audience.
The key factor, in my view, for any customer success professional is their domain expertise. In order to be perceived as a trusted advisor, one has to speak the language of a particular vertical fluently. Therefore, the best certification for a customer-facing CSM is whatever validates that CSM’s domain expertise in the business that the hiring company is in. For example, having a business/accounting degree for a firm offering accounting software. An HR certification for a company offering an HR management system. For a CSM who will be working with customers of a fleet management system, having some certification testifying to competency in fleet management would be of definite value.
You can teach a person all about a software product, Support and Training teams have been doing that forever. But what sets a CSM apart is the credibility, the authority in their voice when they advise the customer on how to get the maximum value from their subscription/investment.
About the only scenario where I could see a generic CSM certification as being an asset in a job search would be if you were going to apply for a CSM position with a customer success management system vendor.