I see where you're coming from and I really agree for the most part.
This has only been the 3rd jam I joined but I dont remember this kind of rate spam being that apparent in jams like ludum dare. I would guess that the 'prize' associated with this jam incentivized this kind of behavior.
As for saying the truth in the feedback, I think it depends on what you're rating. I think a lot of submissions here are from relatively new developers, and sometimes what they need is motivation, not brutal honesty. In most cases, the problems with their games are readily apparent things that they don't need to hear commented over and over anyway.
As for the "cool game, rate mine too" feedback, yeah im not really a huge fan of that behavior (i still rate their game tho cuz i feel bad)