Others have already touched on the subjectivity of the question posed, so I'll try to add something else to the conversation.

If this "average unicorn" were to be found, I don't believe it would necessarily be a case of them being "the most attractive," but there's likely a good chance that, based on physical appearance alone, they would appeal to the most people.

I think this might be a bit of pedantry in relation to the question asked, but given the malleability of attractiveness, and the many factors that go into it, I think it's about the best you can hope for.

There's a small chance that this could be true if the only context is physical appearance, since that's often the "first step" in vetting a potential mate. Once you add other complexities, it starts to lose some of its weight.