If you read somebody’s poem and it makes you want to say, “I think this piece is wonderful; it really made my day, ” just go ahead and say it. Feedback like this is good, but saying why will please them so much better - or it should.
But if you think it’s terrible, be careful how you speak. Some people write as therapy; their life may be quite bleak. Don’t be too harshly critical and leave them feeling worse, but simply go to look elsewhere, and just ignore their verse.
Some poems, though, may leave you with a puzzle or a question, or even make you want to give some tentative suggestion. There’s nothing wrong with doing this – just get it off your chest, but don’t think your ideas are necessarily the best.
With writers, though, who think they are God’s gift to Poesy, if there’s nothing to commend them as far as you can see, you can state your own opinion – of course you have the right – but don’t forget the golden rule: be HONEST but POLITE.