Absolutely everyone is entitled to their opinion and I agree that the opinion was not specifically derogatory.
I understand what a labor of love it is to create something like this and to work so hard on something and give it freely to the community, so I cringed a little when he said he was disappointed in the mod.
Being the consumers that we are we don't hesitate to give our opinion on products, but people should take the time to understand a product like this that they receive for free, they should understand where it came from, why it exists, and how it came to be in their possession.
I have always felt that with a situation like this, where one or more people use their talents and passion to give something to the rest of the community for our enjoyment, for free, that unless those that created it asked me for specific critique, I would avoid saying anything even slightly negative about their creations.
First reason out of respect for the time and effort they put into something I could not dream of doing myself, and the second reason is that I would not want to give anything but encouragement because I don't want them to quit making these awesome things.
My advice is instead of saying "I'm disappointed in this because of...", you could make a post with those same pictures and say "Man this Dirt 55 mod rocks! The only thing that could possibly make this awesome mod even better is if it had my favorite cars from that era, here are some pics, is there a D55 v2 in the works? hint hint

".