I am a member of meetup and run several meetups as well. It is rare for a host to 'host' in the sense of providing pizza to everyone as this guy did. When we have meetups at my house, it is potluck and everyone brings their food ready to put on the table. If it needs heating, that is fine, but everything is brought prepared and ready to serve. Add to the fact, this is a single guy who probably lacks the equipment necessary for serving what you brought.
I don't think the guy did anything wrong and it appears to me that you are being overly picky. Meetups are not typical hosted events and while I would be able to provide a platter, I would be annoyed inwardly that the person did not bring it ready to serve. As someone stated above, it puts extra work on the host who has already opened their home and provided pizza.
I am quite surprised by the TP comment and complaint. He is a bachelor. There was enough TP. Nothing to complain about there.
I think you should rate it positively or don't rate it at all and remember that meetups are very different from hosted events. He went to effort and expense for the pizza. Perhaps a thank you note to him is in order and no negative reviews. That would sound ungrateful to me since he did provide the main meal and venue.