Publishing a paper in a low impact journal when I already have higher impact publications
What does an academic researcher do?  They contribute to academic knowledge by (i) doing new research and (ii) publishing it so that others can benefit from it.  You have an academic work that is not only completed but has already been peer reviewed, with the following verdict


  The findings are still valuable and do offer a minor contribution to the field


So you've made a minor contribution to the field...if you get the paper published.  If you don't get the paper published, you've engaged in almost the entire time and effort of the research and publication process, but without completing the final, key step of broad dissemination.  Since the next thing you have to do is just resubmit the paper -- apparently without any change at all, since


  the paper is fine (in terms of writing, methodology, and results)


from the ground-level perspective of what academic research is all about, it would be pretty strange to stop now.  Indeed I do advise you to resubmit in a lower tier (but still reputable; there are many tiers of reputability) journal.  

You ask specifically: 


  My question is: from the perspective of grad admissions (and possibly future career), would I be better off not publishing this paper? 


No, I don't think so.  Some considerations:


I strongly believe that completion rate of projects is one of the things that separates successful academics from unsuccessful ones -- I know some people who are brilliant and hard-working but somehow never manage to finish anything.  Obviously you are not such a person since you have already published multiple better papers, but still you should be thinking in terms of the time you put into various projects and what the outcomes are.  To work on something for months and years and have nothing come of it is a big problem for an academic.  To be fair, to spend time on something that turns out to be mediocre could also be a problem -- it may be keeping you from doing better work.  But that's a great argument in favor of choosing your projects more carefully; it's not a great argument in favor of bailing out on an already completed project.
I want to add that the above viewpoint is not universally shared.  It is much more common in technical, STEM-like areas of academia than it is in the humanities: e.g. many famous writers have novels that they have completed and decided not to publish.  (Sometimes novels get published decades later, and it gets very interesting: in some cases you can see why they were not originally published, and in some cases you really can't.  Most of the work of Franz Kafka was published after his death by his literary executor, Max Brod, against Kafka's wishes.  These wrongly published novels are some of the highlights of 20th century literature.)  
My own field of mathematics is somewhere between the humanities and the hard sciences in its publication sensibilities.  Many of the very greatest mathematicians publish much less than second or third rate mathematicians.  Gauss's motto was pauca sed matura ("few but ripe").  I am in the same mathematical field as Andrew Wiles, the person who solved Fermat's Last Theorem in 1994 and recently won the Abel Prize in mathematics.  He is 23 years older than I am, I have more papers than he does, and his mathematical contributions are immeasurably greater than mine.  But "even I" do not publish every result that I get, and "even I" sometimes referee papers and see results that I knew how to prove but didn't find valuable enough to write up.  However, I don't think that applies here, because you already submitted your work for publication.  
In the current academic climate, the vast majority of people are served a little better by having one more paper, even if that one more paper is not as good as some of their other papers, as long it is still of sufficient standard to be published in a reputable journal.  If you go on the research academic job market, you will be compared against hundreds of other applicants at every stage.  In order to get a serious shot, there need to be some people in the department who think your work is very exciting, and having one more paper is not going to make much difference there.  However, these people also need to promote and defend you against the candidates that other people in the department are very excited about.  If it's between Dr. A in subfield B and Dr. C in subfield D, after a point it will be hard to tell an argument in support of 
the quality of Dr. A's research from a statement about the relative merits of subfield B versus subfield D.  The department members who are far enough away from these two subfields are likely to take other considerations into account: if one of the two candidates has four very nice papers and nothing else and the other candidate has four very nice papers and two solid ones...well, it's obvious.  So yes, even mediocre publications can work to your advantage.
Early in your career is the best time to pick up an additional publication that is not as good as the others.  Most pre-PhD students in most fields don't publish at all.  Most who publish one paper don't publish two.  And so forth.  Having been involved in multiple projects at this stage provides further clear evidence of your ability to do academic research.  Twenty years later that is not so valuable, but it sure is valuable now.  And the flip side: if one of your early papers is not as good as the others, it shows intellectual growth!  That is an argument for not only resubmitting your work but doing it as soon as possible.