If your aim is to make yourself feel better by putting someone down, then sure, I guess the person is responsible for the code, go nuts.
The aim of any adult should be to get a contribution without the flaws. If that's your aim, then telling someone they are stupid or whatever, at best, does nothing. At worst, they never contribute again.
Hurt feelings?
"Going personal", as the grandparent pointed out, doesn't help anyone improve themselves. You can point out issues with someone's contribution without attacking them, and that's all they need to improve.
If someone is incapable or refuses to learn from that, then you can say that they need to before you can accept contributions from them, but attacking them personally is just unacceptable.
If student is lazy or didn't think his work through - is it personal to call student on that - call him/her lazy? Tell him/her "you're not thinking!"?
Surprisingly - some people take personally critique of their work.
If the work is wrong or bad, explain why. Insulting someone doesn't help them improve.
If you argument is people won't change unless they are made to feel bad, people said the same thing about physical pain - banning corporal punishment would mean no child would learn again. That was nonsense, and so is the idea that making people feel bad is necessary for them to improve.
Recently I worked at a school whose critiques were set up explicitly not to replicate this harshness. I discussed this with a few friends from grad school, and we were mystified: "How can anyone produce good quality work unless they are constantly being torn apart?" "Is there another way to learn besides being humiliated?" Which led us to look at each other and think, well, we certainly don't choose to live that way otherwise.
And that is the question -- and maybe we'll discover on lkml: is there a better way to create something remarkable. And to replicate that behavior in whatever projects we take on.