From day one, we've encouraged her to be curious about objects, and allowed her to mouth different objects. Whether she's a natural mouther because of this, or whether she's a natural mouther despite this, who's to say.
When we try new objects, she will let go almost instantly, but I just click and treat after a second, then two seconds, and so on. I'm going to start her on holding her new dumbbell this week, so you'll see she can be as goofy as any other dog when she starts something new. But we have a lot of fun learning together!
Have you tried making the behaviour happen, by running the object along the dog's mouth and clicking/treating? I do that quite a lot at the start. I might pick up a pencil and put it to Ellie's mouth, and then instantly click/treat, even though she didn't actually do anything but be there. Eventually her curiosity kicks in and she starts to investigate the object for herself.