“Are zirconia crowns stronger than real teeth?”

It almost sounds as though you want to crown teeth that don't need it. This would not be a good idea, in general. Zirconia crowns are quite hard - harder than teeth with no cracks or fillings in them? I'm not sure. It would depend on the circumstances. Talk to your dentist about this, but please don't do anything that is not necessary.

Yes. But zirconia is not always the best for every area of the mouth. It is best for the molars that take the brunt of chewing. Toward the front of the mouth, perfectly acceptable materials in regard to strength often give a better looking result. That is because zirconia tends to be more opaque and not as natural looking in some cases. Dr. Conrad