Is there anything inherently wrong with publishing in a bad journal? There's a journal I read a paper in I wanted to respond to. The journal is open access, has no fee, and I have enough for a short paper (less than like 5 pages, there are no size limits). Writing this would take minimal time away from more important work. The journal is...not great. I don't think it's predatory (it's free to publish and is peer reviewed), though it might be. 

I am a grad student. This response has nothing to do with my main research. 

Would this come to hurt me later? Would people look down on me for having a short, goofy paper in a poor journal? What could they do that's dishonest? Even if they straight up steal what I write, I don't really care that much (though I would obviously not work with the journal again).
It probably won't hurt, but this is the sort of thing you should discuss with your adviser.  Since the journal doesn't have fees, it is unlikely that it is an outright predatory journal but that doesn't mean it is worth responding to. Is this in a field where responses are a normal thing? Other relevant questions: And what do the other papers in the journal look like? Are they absolutely awful or are they just mediocre? That should tell you something about how worthwhile it would be to respond.