Certification is not required. (And I've seen certified trainers with no practical experience and so while they could spout the text, they didn't understand application!) As with everything, a role (trainer) just has to have competence to perform that role. You need to be prepared to defend why you believe the trainer had the necessary competency: this could be through certification or a resume.
Side note: while there are likely some that do need 13485 training, the bulk of training, I believe, would need to be to your application of 13485; i.e., your SOPs.