If I get an article published in a journal as an undergrad, but the research is low-quality, can it hurt my chances for admission to a PhD?
A low-impact publication will not, in itself, have that much impact on your chances of getting into graduate school.  Graduate admission committees will pay a lot more attention to your letters or recommendation.  In terms of graduate admissions, the most important impact of having done research is that it should involve you working closely with more senior academics, who will them be in a position to give you strong graduate school recommendations.

Whether the research you do as a undergraduate gets published or not, what is key is that your research supervisor (and possibly other senior people you may have worked with) will know you well enough to write you a strong recommendation.  If your research was done under the supervision of a prominent, respected researcher, that person's letter will count for more.  On the other hand, if your supervisor has a poor reputation (being known for shoddy or crackpot work, for example), their evaluation of your potential will be weighted less strongly by the people reading your application file.  If the low quality of your publication is a consequence of having worked with a rather poor senior researcher, that is something of a cause for concern, although the best course of action is still probably to get the strongest letter you can from your supervisor, even so.