I don't know the etiquette (although I tend to assume that nobody wants concrit on anything unless they specifically ask and even then I worry...) But your suggested comments are way nicer than the ones that come to my mind which are more along the lines of "Is that text supposed to be readable? Because it's not." or "Is it supposed to be that fuzzy?" and "Wow, that's ugly...I mean, ummm...interesting choice of effects and colors." And on a few occasiona "Canon does not mean what you seem to think it means. Look it up."

But I never say any of that because it would be mean. And also, kind of weird considering I'm not much of an icon maker myself. Coming from someone with your talent and skill, on the other hand, it might be appreciated if you feel out the person first to see if a gentle constructive comment will destroy their soul or something.