Feeding kibbles and bits will not cause ear scabs unless the dog is allergic to something in the food. If that is the case, you need to put her on an elimination diet and find out what is causing the problem. If you cannot take her to your vet, you can research this on the internet and start her on a special diet to see if this is the problem.
If you take your puppy in to be vaccinated, ask about the ears.
The petroleum jelly will provide some moisture to the ear flaps and will make her feel a little better. It is not a cure. You need to find out what is wrong.