It depends on the certification. It also depends (for me, anyway) on the other dog owner/s giving the review.
These certifications do mean something, and require both knowledge and hands on experience to acquire: Dog Trainer Certification
I've also worked with a great trainer with no certifications at all. But she had titled multiple dogs in multiple venues and was highly knowledgeable about dog behavior in addition to having clearly demonstrated knowledge of training techniques through her success with her own dogs.