IMO it's perfectly acceptable to not only not downvote but to ignore a question or answer if you don't know/don't want to put in more effort to figure it out. Maybe someone else has faced the problem and understands how the questioner came to write the question they did?
Many times it is not hard to figure out why the question was written in a bad way. Spending time in the review queue can give a good sense on bad types of content that are typical on the site.
Common issues I come across are very vague statements without an actual question, very broad advisory /opinionated questions, very specific solve this bug in my day job/code , no examples/illustrations added, or a big log or code dump - i.e. no effort to separate out the actual problem statement, do my homework kind of questions, typically there will already be comments asking for improvements without much impact.
I wouldn't down-vote a question that I don't fully understand ( I guess that is true for most users too), at the same time I want to make sure bad content is not encouraged. It is just internet points for both users, nothing to fret about.
