Some of my moms are nervous when people come to look but they never nip. Yorkies can get over protective of their owners and pups. It's up to the owner to tell that dog it's not ok to act like that. If the parents are healthy and she has a health guarantee, and you like the looks of them, I would not rule them out just because of the mom acting like that. I had a mom that acted like she had never seen a human before when people came to look at her pups. I thought, what in the heck???
If you did pick a pup, I would pick one thats not shy and comes up to you. You do have to go with your gut. Do you believe the breeder when she tell you that the mom is not a nipper? A normally easy going mom can act strange when it comes to strangers and their pups.