Others may disagree, but I tend to be pretty minimal in my cover letter, as you will see below. Oh, and you should always write a cover letter.
When I send poems to magazines, I actually don't really say anything about them in the cover letter other than the title. The exception is if it is a special form (a concrete poem or an "action" poem) or I'm focusing on a need they have (ex., I sold a poem to Highlights after reading here they were looking for more "boy" poem; my cover letter said it would appeal to boys, but to girls as well). If I have multiple poems, I list the titles in the cover letter, then attach them on separate sheets of paper.
As I said, I'm a minimalist. Here's was the cover letter I sent that resulted in a sale to Cricket a few years ago (note that I now include my SCBWI membership as well as a very brief list of my other sales).
Dear Editor:
Enclosed, please find my poem, Much More than a Nose, for your consideration for publication.
I have enclosed a SASE for your convenience. Thank you for your time.
Sincerely,
etc, etc.