There are some times when we need to hold the dog's muzzle as technicians - such as keeping the head still while the vet looks into the ears or keeping their head still when the vet looks into their eyes. I do my best to keep my handling to a minimum, use appropriate pivot points to my advantage and I know a lot of ways to adjust myself to keep a vet and myself from getting hurt lol. I'm honestly fairly bad when it comes to puppy mouthing because I find it amusing and don't mind them doing it, or I simply shift my hands into a position that makes it harder to nip at them, so eventually they stop trying.
In general, vets are not behaviorists or trainers. Some of them have more knowledge than others. Those who know GSD would know that they are more of a less is more kind of breed, and you have to work with them rather than against them. I don't know if you as owners have noticed this, but they also always know shepherd people. It is why I get annoyed at times with my co-workers who don't understand my dogs.
I had one of them try to tell me that my new 5.5 month old puppy was picking fights with my 3.5 year old golden. Since I got Doyle, he has been nipping, licking, pawing at Myles' face and body. Myles loves puppies. He loves to play with them. Wrestling is one of his favorite things. However when stuck in a kennel with puppy, he has a limit to how much kissy face he will take. I have been letting Myles tell him it isn't okay, because truthfully Myles needs to tell it. And while he can look/sound vicious, he rarely actually uses his teeth. In my opinion this is dog politics and since Myles is judicious with it, I don't have to be involved. However hearing her say that my puppy was trying to pick a fight had me staring like, are you mad? Other co-workers snorted and said, he has to learn. Some people sadly just don't know dog breeds.
Personally I see it as you have two choices. Either find another vet who knows shepherds better or even just is better at leaving out training advice for those who know more about training, or you can ask the next time you go that she please refrain from trying to discipline your dog as it is your responsibility as the dog owner, not your vet's. I wouldn't dream of doing something like that with a client's dog. Puppies are puppies. I have more fun asking them to sit for treats and squishing their puppy faces, and getting to love on puppies in general.
Just like with your personal doctor, you should be comfortable with your vet and trust them.
Who works for some will not always work for others.