I'd probably see it as if you have something to say, even if it's bad, that means there's some sort of urge/feeling to want to help said game/developer. You
cared enough to leave a comment, even if it's scathing critique. I wouldn't say devs should be thick skinned, we're still people after all, and such comments can sting, but
it is a growing process and we come out the better for it.
*insert long monologue*
read bold for TLDR.
I'm going to just go out on a limb here and make an example out of our game.
If not for some
lovely people (you know who you are if you read this, we love you long time.) that posted
initially bad reviews (and it was really sweet of them to actually edit their reviews after giving the game another chance on their own configured buttons), I wouldn't have realized that hey dude,
no one can see our darn configuration button. (It goes something like this, one person says it, okay maybe they're blind, we'll just tell them where it is. Two people say it, uhh guys, I think we might have a problem? Three people say it. OMG! GAIS WE HAVE A PROBLEM, FIX IT NOW.) Whelp, thanks to that we were
able to touch it up if only a little bit just a mere twenty mins before the final submission the other day.
What they said, probably
helped solve at least 80% of the complaints about the config, in the sense people have now noticed said button and aren't ripping our heads off for it and are actually enjoying the game. And now in a version that we're working on

it's available... EVERYWHERE
(instead of just the main and level select menus) in a pause menu that we've finally implemented
(yay!). Will have nothing left to say if people don't notice it this time XD.
I must say we've been pretty lucky in terms of the comments left behind for us most for most of the part. Alot of it has been constructive and fired us up
(I mean it's even fine when a comment hurls an insult at a feature or something, we just make light of it and go "Oh yeah? We'll fix that. See how you like it now!") to cut our break short
(we had wanted to crash, burn, relax and play for the whole week lol) that we started getting back into the swing of things the day after the submission. I've looked around in other projects and have noticed some haven't been so lucky yet but eventually, as long as they worked hard on it, they're bound to catch someone's eye.
It's easy to tell when effort has been put into a project or not, and those are the ones that deserve every bit of critique good or bad so that they can improve/know that someone actually played their game.
Bottom line is,
people cared enough to leave critique that can improve your game, that's golden free advise. At least you can
tackle the problem now, rather than in an actual fully published finalized release.
The only time I'd say
don't comment is when you have personal issues with the developer themselves then that just turns into a personal attack/harassment thing. Deep inside, I would think even if it's a bad remark, I'm sure every developer wants to know, because at least
something said is better than silence (then that becomes like an unspoken statement which makes the developer think, is my game really that crappy that no one at all is going to comment?).
The other time I'd say to
not leave a comment is when you have the ulterior motive of flashing your own game around. It just makes you look horribly shallow as a person/developer. I've been seeing some people do that in almost every other project page amongst anything that's trending/high on votes, and it's just really really sad. There's even one that leaves a bad/unconstructive/sandwiched 1 liner every. single. time. then flashes their own game.
There is a GIANT LIST of games. We know how to find your game if we want to play it. (If you read this, do you honestly think, other developers/fans would want to give a fig about your project when you're going around to every other project and just being the equivalent of a very loud chihuahua(just analogy here, I love dogs) barking look at me, all these others are terrible! Think beyond this competition, with this kind of behavior, just what are you trying to tell people about yourself as a creator?)
However, it's good that they bothered to comment on the project he was advertising on in desperation of hey play my game!
But there are other and better ways to do that in a not so annoying manner as opposed to another that went around just linking his game with no comment at all <- this one received more scathing remarks for general a**holery.
Finally,
almost every game has it's own golden moments (cause let's face it, there are some made with no ****s given). So while you might have mostly bad to say. I'm quite sure there'd be at least something good to talk about, as people have mentioned above like a joke that made you laugh. It could even be the art if the game itself was terrible. Or the music, or sound effects. There's always something to talk about, unless say it was a pixel. A very magically uninteresting teeny tiny speck of a pixel on a giant canvas which is the player character, there's absolutely nothing to say about the lack of effort there.
End Monologue =X
Edit: Ermergerd. I didn't realize that was sooooo long 0_0