I sometimes ignore poets who come in for a hit and run critique on their poem without offering any feedback of their own. But that doesn't outright harm a poet. It just means that those who give more tend to get more.

So I'd encourage anyone to post a poem for critique, but if you want to get the broadest number of critiques, it's best to become part of the community, at least for a while, that you wish to do you the favor of critiquing your work.

And it is a favor. "Duh, what?" or "Okay, cool." isn't much of a critique, but some people put in much more effort than that for a stranger. You may not be able to pay them back, and it probably isn't their intention that you do, but you can pay the forum back by helping someone else in the exact same way you'd hope to be helped. That will keep the crit forum a place where people like you can come to have a good poetry experience with a wide variety of helpful people.