Groomers at Petsmart do NOT have to be licensed unless it is state law. There are many states in which no groomers have to be licensed, and the ones at Petsmart in my area typically are not.
There can be good groomers at Petsmart. However, keep in mind a few things.
1. Petsmart is a training company. More than half of the groomers at Petsmart (and Petco) start as bathers or cashiers or something else, and are sent to their corporate "grooming academy" for a couple of months before being allowed to groom. They are then required to work 2 years before they can leave Petsmart. Most of them leave after 2 years in search of a better place to work. So many of the groomers you will find there have been grooming dogs for less than 2 years. Many Petsmart groomers are also very young- they can legally start grooming at 18.
2. Petsmart is a corporate, money-driven business. They want your money. That doesn't necessarily mean they won't do a good job. It does mean that they will book every single kennel in their entire salon if they can, and your dog may be one of 20, 30, 40, or more dogs being groomed that day (The one I interviewed at wanted each groomer to do at least 8 dogs per day, and no less than 8 groomers worked in a day- that's 64 dogs every day.) That's a lot of noise, a lot of activity, a lot of risk of fleas, ticks, and contagious diseases compared to smaller private salons, which often groom less than 20 dogs per day.
3. Petsmart caters to pet owners, which means they do not typically teach a lot of "fancy" clipping. Most of the groomers I've met who worked at Petsmart are big fans of pretty much shaving every dog down and giving it a generic head shape. If that's all you want, you're in luck. If you want your dog to have longer hair, a more unique appearance, or a specific idea you've come up with, you may run into problems.
All that being said, I have known some fantastic groomers who worked at Petsmart for 20+ years. I've also known some idiotic teenagers who couldn't tell one end of a dog from another. I am a professional groomer myself, and having seen just about all sides of grooming (Petsmart and other corporate shops, private salons, mobile grooming companies, vet clinics, etc.) I would not take my dogs to Petsmart to be groomed. From what I've seen, if I needed a groomer, I would find a small private salon with no more than 3 groomers on staff. These types of places are typically quieter, cleaner, and their groomers are often much more experienced and knowledgeable.