Is it appropriate to email a philosopher in Academia about a journal article they may have published, whether it be with a possible criticism or query? This is assuming that the emailer is just a casual reader of said journal, not an academic, with no involvement in a university. Also assuming the email is polite, to the point, not of an exorbitant length, and the emailer is quite sure of his query or criticism.
Absolutely! Readers of such articles usually number in the dozens or, sometimes, hundreds, and very few readers write to the author. So it's really not a burden and, often, a really enlightening experience. You'll find that, in nearly all cases, you'll get a friendly and grateful reply.