Dear Mag Writer…
Presuming the magazines are editorially discriminating and of professional quality, I support mentioning them in the credentials portion of your query letter. You’re seeding confidence in your professionalism and your writing chops. Ring this bell extra loudly if the magazines’ subject matter jives with that of your book—this establishes your expertise with the topic. A caveat: If the magazines are political or in some other way topically sensitive, only mention them if the topics are relevant to the book you’re pitching.