I don't think I would call it 'wrong', per se, but it's probably not a good idea. If something makes you uncomfortable, you do have the choice to refuse it. Accepting the commission and then talking negatively about it where you know the commissioner will see it is very passive-aggressive, and I would likely avoid artists that seemed to make a habit of it, for fear of being given a negative image in return for daring to commission them.

Overall, it just seems like poor business sense. I could easily see commissioners reading the artist's comments and going, 'Well I don't want to have to worry about that happening to me, so I just won't commission them...'.

If you think of the artist's gallery as their advertising, saying "I draw these things and this way", commenting that they don't want more of a given subject matter makes sense. It's the other side of "I loved doing this."

Is it unpleasant for a commissioner to see? I think so.
Should the artist take a commission they aren't comfortable with? I don't think so.
Is it smart to post art of a type you never want to do again? I don't think so.

But the bottom line to me is that it's their gallery and their right to say what they wish.

If a commissioner doesn't want to risk the comments, they should avoid the artists that make them regularly. It's not like there is a shortage of artists at every price and skill level out there.

I've seen something like "This work is a commission and the artist does not necessarily support/condone the subject matter" float around sometimes. Which I think is fair and valid. You don't need more. Networks post shit like that before talk shows and infomercials. I think it's perfectly acceptable. But nothing like "I find this super skeevy, but here it is"

If you object to it that much, don't accept the cash, and refuse the commission. Otherwise I'll side eye the artist because hey, seems your morals are for sale, but you'll make a stink about it. That's just tacky.

Personally I find it tacky to do so. I'm being paid for a service and these are my clients, I show them respect. If I hate an idea so much I either won't accept the commission to begin with or I will and either post it as is without commentary or choose not to post it to my gallery at all. I guess I feel like writing something off-putting towards the commission/idea is akin to biting the hand that feeds you, I find it in poor taste and sort of rude or unnecessary.

I think it's pretty rude IMO if its a negative opinion. I have a friend who commissioned a big name artist and the description the artist wrote something akin to "yeah, I don't know either :P" regarding the subject matter they didn't find appealing and being really dismissive of it. It was actually pretty hurtful and in some cases can invite similar opinions in comments, detracting from the art and turning it into an "I don't like X" party.

I tend to follow the "if you can't say something nice, don't say anything at all" approach, or, if you don't like the subject matter, don't take the commission!

I find it extremely tacky and I wouldn't commission an artist I saw doing that in an open/public space. If you don't like it, don't do it. If you choose to do it anyway, keep your griping to yourself or your private online spaces.

I think that's totally unprofessional. Something makes you uncomfortable? Don't take on that job, it's simple.

It's not very professional, that's all I'll say about it.

If I didn't like working on the piece or I had problems with it, the commissioner never hears about it' they're not paying me for my opinion. If I need to vent, I have an artist buddy I can vent to (or even better, bounce ideas off of to get out of an artistic jam.) If it's something I'm not comfortable with or don't like, just don't take the commission.

I will only add in an additional comment if I have something positive to say ("I loved drawing this character's accessories" "I really liked the prompt I was given") Otherwise I keep my mouth shut.

As someone commissioning a piece, it's really insulting to be super psyched about the piece you got... and then reading the comment from the artist about how they didn't like doing it.

Honestly, I think it's pretty rude to do and I'm a "fetish friendly" artist that will take on content that doesn't always reflect my personal tastes. I think it's okay if an artist chooses to politely express that the content of an image doesn't reflect their own personal tastes/opinions, but outright making a statement about the subject matter or adding a snarky quip seems incredibly disrespectful to the commissioner. It almost gives off the vibe that now that the transaction is complete and the money is in hand, the client suddenly has no value to them.

I understand that artists may sometimes take on work they're not always comfortable with; When I was establishing my self I often would do that because I couldn't afford the luxury of denying requests, but at the same time I always made the choice to not post the stuff I wasn't comfortable with. Artists are not obligated to post anything they don't want to, and instead of mocking their client in the comments it's better to just not post it at all.

Personally?

In my opinion - it is easier to just put down generic info down on my commission pieces no matter if I liked it or don't.

I found when I put I liked doing something - I had another commissioner ask me if I didn't like doing their items. Then, feel guilty when they think I didn't.

I only put my personal opinion on my own items.

I feel it is more professional that way.

I've put a disclaimer if I don't particularly like the subject matter (like "this is not my cup of tea") which was apparently necessary because people were saying "I" was hot in said pieces of work. :/ I think it's normal to mention if it's not something you like as long as you don't trash-talk it.

It's a poor business decision, overall. If you say that you don't like it, or you don't normally do things like it, or that you didn't like the outcome, it appears as if you aren't doing the best job you can do.

Like Anuvia said above, in issues of fetish work, it's often better to just not post the piece, rather than have to make a disclaimer that could seem rude. I personally just finished a reference sheet commission that focused on a topic I don't care for. I chose not to post it, rather than potentially make my client feel like they pressured me into doing something I didn't want to do.

It's usually better to just leave neutral or positive comments on pieces, and avoid the negative stuff.

Honestly putting down anything that isn't "OMGLOVE" opinion wise will bite you in the butt. It's happened to me. Someone sent me a huge description, with full history, like pages worth, then wanted the character just standing there. I was vastly amused, as it means the client loved their character, but when I mentioned it in comments the client flailed. Took me a while to assure the client that wasn't a bad thing.

So if you want no backlash, even if you mean well, keep your opinions out of comments.