This premise is a bit precarious, really.
If you are not specifically asked to comment, and you don't like a piece, then move on. To do otherwise is often crass and inviting of criticism and retributions.
Silence is a very powerful motivator for a lot of people.
Anyone with a shred of intelligence, that posts an image they're proud of and receives zero feedback but also sees others actively commenting will realize their work is missing something... including an audience.
If you are not asked, but it is implied or simply like this site where you have the opportunity to comment, you are still best to walk away from the piece, or collection, and let the view-count versus silence speak you. This also leaves room for others to step in.
If you are specifically asked, then honesty is the best as long as you are respectful and, above all else, realize you are not the keeper of all that is aesthetic.
If you believe the sun will go out if you sit down, then you are best to walk away in all cases else bear the wrath of others.
If you feel compelled to comment, and you just cannot walk away, or it is a new person, a fresh upload of their latest work, then you need to step back and look at all the work, or a representative sample, before commenting.
There are extremely few instances where it is, in my view, useful or appropriate to tactfully state the work is not to your taste.
A scenario might play as follows:
"Of the work you have presented I don't find this example to be your strongest or nor to my taste and aesthetic. In real life I've not been attracted to large piles of fresh fecal material and I'm not familiar with anyone other than my dog that is.
That said, you've done an admirable job of accurately exposing and rendering their likeness so if your goal was for a visceral response, then you have succeeded well.
If that was not your goal, then you might try applying your skills to the creators of this material, preferably biased to the intake end, while going about its or their day. Failing that, you might try different sources and situations for more fertile material"
There are caveats because there is usually someone, somewhere, that is either ignorant or itching for a fight. Indeed, some people make a hobby of classless rhetoric, abuse and more.
Other instances exist that don't always fit either. Typically those are situations where differences in language, culture and/or social abilities are foremost. Those are all independent, by the way. Furthermore, some days people find fecal material in their cornflakes, don't have a camera at hand to document it and it all goes down hill from there.
I regularly see work on this site and elsewhere that is either not to my taste or demonstrates any number of flaws and weaknesses. But I am fully aware that the few images I have posted include some degree of deficiency. After this many years, I've managed to learn how to reduce that number.
More common, though, is for more work to be posted that is decent to exceptional. Those are situations where divide and conquer is a good approach where "there is room for improvement".
All in all, if someone takes the time to post a work or comment, they deserve respectful consideration. Personally, I prefer to hear people articulate what it is they like, or not, about a piece or why they think it works so others can better understand why certain choices were made.
To close, it is important to realize that perceptions and perspectives change thereby changing how we react to our environment. Some images take time to understand or appreciate so a first response may not be indicative of what you may think days, weeks or months into the future.
Of course there are also situations where the subject matter is rhetorical, humorous or spoofing. The work you have deemed unpleasant and plan on commenting may later be discovered to be bait you fell for it with serious commentary and the like.
More could be said, but I'll leave space for others to comment upon later.