We all know that downvoters on TGO and any SE site are a fact of life that has to be accepted. Unless there is a pattern of serial downvoting, complaining is pointless, and can sound whiny.
However, Meta is different. The whole point of Meta is discussion about problems, or at least perceived problems, on the site, and the participants are almost exclusively frequent users of the site.
Thus, it is important for downvoters to give a short explanation of why they downvoted; if not, how can the discussion be furthered and the site improved? I have no problem with users seeing a question or an answer on Meta, saying "meh" and moving on without voting or commenting. But a downvote is a lot stronger than "meh" and explaining it, even with one word, is a service to TGO. For example, you think the Q is about something so trivial it should not be discussed, downvote and comment "trivial problem" or some such.