"Which animal is the strongest?"
Translation:Które zwierzę jest najsilniejsze?
I don't think so. I can't say why, but "mocny" is just a lot less commonly used than "silny" and it has its collocations, outside of which it just looks strange.
Somehow that would make more sense to me if you were talking about Pokemon, not real-world animals. Although I'd still go with "najsilniejszy" even then.