Just seeing this now after already having submitted some ratings and comments. I agree fully with your approach to the stars and it's what I've been doing. I'll use 2/3/4 stars mostly for when I have significant criticisms, minor criticisms, or nothing but tiny quibbles. I'll give out 5 stars very sparingly, and 1 star only if I don't feel like the person bothered to try, e.g. wildly irrelevant theme or the "design" is just a text file pasted into a trifold template.

I disagree about offering constructive criticism in private, although if you insist on it I can delete the comments I've already made and send them privately. I have a few reasons for wanting to do it publicly, though.

Firstly, because I think feedback isn't only useful to the person who made the trifold, but to the community as a whole. Secondly, because I don't think anyone should take too personally whatever criticism they receive for something they did under the constraints of a jam—we're here to get the creative juices flowing and hopefully learn something. 

Finally, because I think the dogpiling you're worried about is more likely to happen with privately submitted comments... assuming people are trying to act in good faith and be helpful, you won't get people reiterating at length what others have already pointed out, they'll just say "I agree with what has already been said about X," and then move on to their unique observations. Whereas if we were all going with private comments you might easily get 30 messages pointing out the same issue.