You have to make the decision. That's the idea that when everyone gives their subjective ratings, the result should be closer to an objective answer. If you believe it's 1 star, you can put it. Someone else would put 3 stars, then it would average out to 2 stars, for example. Do you see what I mean?
Yes but I think it's a bit different with assets. If you saw a game that had great textures, but then found out that none of them were made for this jam and instead borrowed from somewhere else, what rating would you give? In a sense I would be tempted to give only 1 star, but that would bring their average overall score down as well, which is unfortunate, because assets aren't disallowed and when they made the conscious decision to use some, they most likely didn't expect to get bad ratings from it.
That's the idea that the overall score will be averaged out to a more objective score. The more people vote, closer to the real mean it is. This is based on statistics, so just make your best subjective decision and put what you believe is fair. If we wanted to follow specific guidelines for adding a rating, we would go with judges.