This should not only be allowed but strongly encouraged, despite looking not very honourable at first glance.
If all participants were to cast a vote on their own pizza, that would even things out and would effectively have no impact.

But if voting on their own pizza is disallowed or frowned upon, then it discourages participants from voting at all (i.e. on other pizzas) as that would be acting against their interest.

Maybe for the next edition, we could have a rule that you can vote on your own pizza, but only if you also cast votes on other ones.