Telling a person their work is not up to standards is not belittling. Now if you tell the child "you are a piece of crap", that's a different story. But telling them they played poorly--that's the truth. Same thing with work--you can tell me my work was bad, but you can't tell me I am bad. If you do, then I will find another job.
Yes, the coach could have celebrated that they had any points at all. But the coach is pushing them to do better. Rec League doesn't mean goof off the whole time. And 6th grade is too old to be learning just the basics. I wouldn't expect high school plays, but they need to be able to do something. I don't let DS turn in poor school work just because he thinks it is great. It isn't by the teacher's standard, or mine. And I am not going to instill false confidence in him by telling him he is doing a great job when he isn't. I praise his effort regardless of the grade. But he has to do his best. He is getting a C in math and with his dyslexia--that is awesome!! But I expect him to get an A in Social Studies and Science because he knows that stuff backwards and forwards and if he isn't getting a good grade it is because he isn't doing the work.
And no, I don't expect adults to take care of my children all the time--not at the older ages at least. My kids have to learn to take care of themselves too. My job is to buffer the world, not prevent it from happening to them. I had coaches who yelled like crazy--because they cared and they expected the best from me. I had a riding teacher who I feared more than the jumps AND I knew she would never ask me to do something I could not do. So she had me jumping 4 feet and I won jumping classes and I loved it.
At what point do you learn to deal with mean people in authority? In college? Your first job? I would rather coach my kids through a workable situation with me there to support them vs. them having to encounter it on their own for the first time. And you sure as heck can't call their boss and say your child should be treated with respect. You need to learn how much is too much and when it is too much, how to cut ties gracefully and move on.
The bottom line is that the coach doesn't sit right with the OP--there are plenty of other leagues out there for sports and I would support the OP finding another one. Just know that it won't be perfect--hopefully it will be a better fit.