All feedback is useful, even if all it tells you is who you're NOT writing for...
It pains me to see stories, especially in later divisions, with no or few responses. Rather than read a whole division, I often select one of these (fewest views, or fewest responses) to read and comment on.
In a peer voted contest, for me, the feedback is the main reason to enter, unless you're the sort who thinks you're going to win(!). Not that you should send in something rough, as the feedback will probably all be the same. I'm quite happy to suggest the story doesn't fit into the allocated space - this isn't a fault with the word count limit, it IS a fault with the story as is. (But "I wanted more" is valid, and appreciated!)
That all said, authors aren't made of stone. Criticism should be, as far as you can make it, helpful, rather than just sledging into a piece. Say what you like, as well as what you don't. And don't fall into the trap of taking it over ("if I was writing this, I'd have...") - it's NOT your story.