Essentially, there's nothing wrong with it.
I recently came across a meta post on Stack Overflow about what the OP calls "bandwagon voting thru chat". The accepted answer (with a score of 19) says:
People coordinates all kind of efforts through chat. If you make any kind of behavior "fraudulent" you should also include flagging spam, voting to close, voting to reopen, voting to delete, voting to undelete, etc.
That people are made aware of an specific post that merits attention trough [sic] chat is not in and on itself fraudulent. You can ask in a chat room for voting in an specific way for a post, that doesn't mean that everyone will do as you ask.
I agree with this: there's nothing inherently 'wrong', at least as far as the system is concerned, with bringing people's attention to a low-quality post through chat. However, that still leaves the important issue of personal feelings: how will somebody feel, knowing that their post has been singled out for mass downvoting/close-voting in this way?
"Vote for the post, not the person."
An important mitigating factor is that we should always be voting on the post based on its own quality, not on who posted it. Downvoting riddles just because they're posted by a newbie is at least as bad as upvoting maths problems just because they're posted by Gamow.
Unfortunately, this can be a difficult habit to overcome: since the quality of a puzzle is often difficult to ascertain until it's been solved, it's easy to cast votes based on a thought process like "I know this person, they always post good puzzles, +1". To try to break this way of thinking, I suggest imagining that the puzzle was posted by someone else: e.g. if you see a newbie riddle, think how you'd vote if it was posted by Hugh Meyers or Alconja.
Anyway, I'm getting a little off track here ...
Always be friendly.
This is the most important thing in pretty much any interaction here on SE. Be nice, assume good intentions, and try to make it clear that your own intentions are good too. When discussing a low-quality puzzle in chat, remember that the OP may see your conversation and try to be tactful: don't say "this is crap", say "this has too many possible solutions" or whatever. Constructive criticism always hurts less, and is more likely to be taken to heart and used by the OP to improve their puzzle.
In fact, don't just act as if the puzzle writer is reading your conversation in chat: actually post a summary of it in a comment on their puzzle! Again, constructive criticism can help the OP to improve their puzzle, but only if they're actually aware of the problem. One of the guidelines laid out in A Theory of Moderation, which as a moderator I've always tried to stick to, is:
Whenever possible, try to leave frequent comments on posts where you've taken (or considered taking) a moderator action, explaining the reasoning. This is important so that community members can learn the norms of the community and the moderation policies.
This doesn't just apply to diamond moderators. On SE, everyone with enough reputation can take "moderator actions" such as voting to close or delete a question. When you do so, it's nearly always great to explain your reasoning so that the affected user can learn from it and improve.
So if a bunch of people do talk in chat about a particular question and encourage each other to downvote or VTC it, it'd be great if one of them could write up a summary of their conversation about why the question is bad and post it as a comment on the question.
Of course, all this describes ideal situations which won't always happen. Everything is easier in theory than in practice; every battle plan is perfect until the first blow is struck. But the guidelines above might help people to stay nice and friendly even when downvoting or criticising each other's puzzles.